Bash while read line redirect i'm learning bash on linux, i found script for "while loop reading from a file" and modified a bit as below, i 'm trying to read a list file containing all the git repos, and i wanted to Can those questions maybe be merged somehow? Both have some really good answers that highlight different aspects of the problem, the bad answers have in-depth explanations in the comments what's bad about them, and as of now you cannot really get a whole overview on what to consider, from the answers of one single question from the pair. You should use a while loop with the read -r command and redirect standard input to your file inside a function scope where IFS is IFS= and -d '' causes all of stdin data to be read into a variable indata. the console before launching the desktop environment), then the tty path will look something like /dev/tty1. Share Improve this answer You'll have both the input to the loop and read, and the output from echo connected to the same file, yes. I am trying to read from a source txt file in bash and I want to ignore the first line which is the column. If you don't want to use REPLY, you can simply set IFS to a null string to In this version, at the end of the loop we specify multiple input redirections using the full general form of bash's input redirection: [n]<word. cat file | while read ROW do done because the pipe creates a subshell and makes me lose the environment variables. /testScript , which reads from . – According to this bash redirection cheat sheet, this syntax passes the output of cut -f2 file. @Oliver: thanks for your comment and your downvote. bash read stdin after piped input. The line is split into fields as with word splitting, and the first word is Using “read” Command. I made my scripted to process GPS data. . In my script I reading line by line, checking how many fields have been populated on that line and determining how many commas i need to append to the end of that line to represent all the fields. txt . You can redirect your used script commands in different ways whenever I have a text file that can have X number of fields, each separated by a comma. The read command modifies each line read; by default it removes all leading and trailing whitespace characters (spaces and tabs, or any whitespace characters present in IFS). Unfortunately, programming is not simple. In real life, __process does some sed on the output + some logging depending on presence of The problem is that do_work. x. While using the `read` command, there are several best practices to keep in mind: Mind variable scopes: Be cautious about variable names, especially if they may conflict with existing variables in your script. You wind up just reading the first line of the file over and Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company I'm using a "while" loop within a shell script (BASH) to read line of a file (one by one) -- "Fortunately", its not working the No. who got me on the right track. In the last line, the echo command echoes Don't store the output in an variable/array (you can get all sorts of buffering issues in some cases--although not with /etc/passwd, and there's simply no need), just do something like parsing it line by line or use a tool like awk or cut as perreal suggested. bash; Share. If we we're on a native terminal device (non-pseudo) meaning the backend is hardware or kernel emulated (e. Alternative solutions: Eliminate echo. Read from STDIN and output to a file. /predate. What might be happening is that read is designed to work with text files. (without redirecting it works fine). This command processes data from files or standard input, making it an essential tool for automation and text manipulation in Unix-like operating systems . In this example, the filename will be taken as a command-line argument. That invokes as few cat instances as necessary for an arbitrarily large number of . sh runs ssh commands and by default ssh reads from stdin which is your input file. Now with your example, awk is already splitting on spaces, so IFS doesn't make much of a difference. It's not the braces. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Visit the blog if you are using a variable name in the shell command, you must concatenate it with + sign. The canonical way to run a command for each file found by find is. You can use the read command with a while loop to read from a pipe within a bash script. -type f -exec cmd {} \; while read line; do done When you call . So it opens the file, runs read, then closes the file every time. Calling it in a loop is almost never required. txt Don't use Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company That's because the condition of the while (actually a list of commands) is executed and evaluated on each iteration, including the redirection. Mac OS X 10. But when you do while read < filename; do done, the command is just the read. First, create a script which will add the timestamp to each line (called predate. Stderr is not—it's written after every '\n'. The condition of a while loop can be as complex a command as you like. I am trying to bash script to take those two inputs line by line to test into a program. txt , your while loop will read from its standard input, which is inherited from . 8. Should you ever find yourself on a restricted system with /tmp being read-only (and not changeable by you), you will be happy about the possibility of using an alternate Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company BTW, in a real-world scenario, I'd write this operation more like find . while read var file; do ; done < file Basically, while read var; do ; done < file will read each line of file and save it as var. file" while read line do echo $line done < test. Can anytone tell what's wrong and how to fix it? First argument is a path, $ echo > $1 bash: $1: ambiguous redirect Share. Could you please suggest how to fix this or an alternate mechanism? The while read loop is a powerful structure in Bash scripting that allows you to read input line-by-line. The problem arises if the file doesn't end with a newline: last line is not read. e. Learn more Explore Teams And here you can also use a process substitution if you want to get the data from a command: while read a b c <&3; done 3< <(echo $'1 2 3\n4 5 6') (process substitution is a bash/ksh/zsh feature) 3. # use split+glob (leave ${lines} unquoted): for line in ${lines}; do echo "${line}" done Note how I set the IFS to not split on line. conf > $tempconf The content of the test. echo-e "$ line \ n" . Test your scripts: Verify that your prompts and input parsing behave as expected to avoid user confusion. conf is: I know that (1) I can loop through lines of a file with while read line do something done < file (2) I can pause my program with something read -p "press any key to continue. and. There are tools you can use to capture output from a command while still providing a tty for it: unbuffer, script, expect, screen, tmux, ssh, possibly others. Correct text files must end on a newline. The here string <<< is a bash extension. Next, the echo command echoes the quoted message. So, the read command takes its stdin and sets the VAR variable, and then its subshell exits taking with it the variable. Redirection is a process that allows you to change the default input source or output destination of a command. This method requires some preprocessing to translate newlines into spaces (using tr) and to merge several spaces into a single one (using sed): { tr '\n' ' ' | sed 's/ */ /g' | while read -d ' ' WORD do echo -n "<${WORD}> " done echo } << EOF Here you have some words, including * wildcards that don't get expanded, multiple You can use a named pipe as an alternative way to read data from a child . This can be observed via tracing system calls with strace command. This is discussed more fully in the following places: In bash, read after a pipe is not setting values, Pattern_File is the name of the file which is placed in the same directory as the script. txt file line by line to another . sh Successive 'read' operations omit leading characters from the line. Tour Start here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site The content of the script is: #!/bin/bash tempconf="/tmp/test. , shell i'm learning bash on linux, i found script for "while loop reading from a file" and modified a bit as below, i 'm trying to read a list file containing all the git repos, and i wanted to There are two ways of reading a file line by line that I want to discuss here: echo-e "$ line \ n" . If all you want to do is echo the input back to output, cat does that already. txt | while 3. lines=$(cat <<EOF one two three four EOF ) IFS=$'\n' # split on non-empty lines set -o noglob # disable globbing as we only want the split part. currently I have a file named testcase and inside that file has 5 10 15 14 on line one and 10 13 18 22 on line two. What you need is a trick to pipe stderr through another program that can add timestamps to each line. Hello from the future world of 2022! This is best written (in Bash ≥ 3 and possibly even older) as while read; do let CNT++; done <input. txt so i must assume there is something more interesting going on so i have put the two options that solve the question as asked as well:. csv isn't an argument to the script -- it's an instruction to the shell about how to configure stdin before it starts the script at all. E. It might be better if you just ensured that it was LF-terminated before processing it. So, when you do . Very often the processing could take place in an Awk script and the shell while read loop just complicates matters. Follow answered Jun 11, 2015 at 7:46. However, by specifying a small integer in front of a redirection we can redirect multiple input files to the command, in this case the command is the while loop: Tips and Best Practices for Using Read in Bash. /testScript < . Dash does not have the <<< redirection operator. Can (I cannot add an answer to the question, hence adding as a comment) If you just want to extract the first or last word from (eg) output from a command, you can simply use the shell variable string substitution operators, to remove the first or last section of a string. $(. Tour Start here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site Simple approach. My favorite solution to this is to pass the file over something other than standard input, like file descriptor #3: while IFS= read line <&3; do # The <&3 makes it read from FD #3 done 3< "${CONFIG_DIR}/queue" # The 3< redirects the file into FD #3 @ata Though I've heard this "preferable" often enough, it must be noted that a herestring always requires the /tmp directory to be writable, as it relies on being able to create a temporary work file. A here-document is an input redirection using source data specified directly at the command line (or in the script), no "external" source. csv as an argument, by running You can't use the return code of read (it's zero if it gets valid, nonempty input), and you can't use its output (read doesn't print anything). sh') and avoids all the If you really must use Bash to handle text lines, it is normally very recommended to rather do it like in: (showing only the first line of the loop) while IFS= read -r line ; do unless you really know that you need otherwise. The syntax of a for loop is different:. There are two things you can do here, either you can set IFS (Internal Field Separator) to a newline and use existing code, or you can use What's happening is, the read command fails when the input is not terminated with a newline. I am passing the /etc/passwd file as the input to the while command. <random string>, while in dash they are implemented as anonymous pipes. Here the read command As I understand the bash documentation both | and < redirect stdin. After shell expansion it is expanded to: Note that the file reading or writing positions are also duplicated. In Bash, reading files can be achieved using different techniques, and one of the most popular and versatile methods is using the while loop. txt Note that it'll take care of situations where the input is piped from another command, and not just from an actual file. Anything else is considered part of the command line. The echo command writes the line of text in the terminal window. So a file is read, "processed" then the resulting line oriented data is used in a while read loop. find . Replace bash with sh to see how /bin/sh performs this redirection. So first version works fine but I don't understand the Explains how to use a Bash while loop control flow statement under Linux / UNIX / BSD / Mac OS X bash shell with examples. Read File Line With “while” Loop. log file must be filled while process is running) @troelskn: the difference is that (1) the double-quoted version of the variable preserves internal spacing of the value exactly as it is represented in the variable, newlines, tabs, multiple blanks and all, whereas (2) the unquoted version replaces each sequence of one or more blanks, tabs and newlines with a single space. txt) is a command substitution attempting to invoke the command . My script is as follows: #! /bin/bash # It's called a pipe. -maxdepth 1 -type f -name '*. I have these requirements: prepend every output line with a string (e. As @Zac noted in the comments, the simplest solution to the question you post is simply cat file. The clear intent here is to prevent do_something from reading from the sample. txt isn't an executable file that produces a list, but instead a text file containing: $ cat input. To retain the value of first_line you must not run the read in a subshell. Explanation is in Blue Moons's answer. , if I enter When you pipe one command into another on the command line, like: $ foo | bar The shell is going to set it up so that bar's standard input comes from foo's standard output. For clarity, this command will output a few million lines of text and it may take roughly an hour or so to do so. To read a file line in bash, use the read command In this guide, we learned a few important scripting concepts such as what is Bash redirection, what is file descriptor, how to redirect an output to a file, how to handle stderr, what is /dev/null and how it is used in Bash In this article i will show the general syntax of the while read line construction in Bash and an example of how to read a file line by line from the Linux command line. In bash these are implemented via temp files, usually in the form /tmp/sh-thd. So, when you try read yn, it reads form r again. sh gets you to the right place in a way that works even with files with oddball names (touch $'foo\nbar. of times the file has lines in it. Zoltán Now available on Stack Overflow for Teams! AI features where you work: search, IDE, and chat. data, which avoids the pipe, and thus variables losing their values when that subshell Redirection in Bash. all only take one word as their argument, and use that as a filename. Also, globbing characters aren't terribly common in /etc/fstab, so you're probably safe there. Do I have to install anything else there? Yes, actually install bash and then run your script in bash. To read a file line using a while loop, incorporate the read command with a while loop. While the redirection after done happens only once for the whole loop construct. Using the “read” Command in a “while” Loop. Once read detects EOF, it stays at EOF; it won't spot additions to the file after that. This is covered in the Bash FAQ entry on reading data line-by-line. I know you can use the following to compare a list of strings from inputs. In a POSIX shell, any command sequence involving pipes ( cmd1 | cmd2) runs both in subshells. Here's the Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Visit the blog The problem is that do_work. Improve this answer. I read here that calls for 'read' input within a 'while read' loop can cause problems, and wonder if the ffmpeg call is doing something that confuses bash, but this seems unlikely and bizarre! NB: I don't really understand the ffmpeg call - I got it from an answer to another I assume your shell is bash. As a result, you only see the first line processed, because the command consumes the rest of the file and your while loop terminates. awk reads each line and acts on it by applying the rules you specify. You should do either one of two things, but not both:. Bash prints ambiguous redirect error, because it can't parse the destination. 3}" done < users. So when you run this: cat . It seems obvious to me that making the same types of changes to the line: exec 0<inputFile vs. When the buffer is full and wine attempts to write more, the wine process is stopped by the kernel. Can someone point me I am trying to report lines found using grep and while. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company While loop with input redirection and read command. txt > output. The read position of the input will be left where it was, exactly after the latest line read. The redirection from the server list file applies to Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company The way bash runs jobs, the first link in the pipe-line becomes the group leader (as you should be able to verify with ps). The -r option is used not to allow backslashes to escape any characters. for name in word; do list ; done where word is usually a list of strings, not a command (although it can be hacked to use Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Visit the blog . The way I want to do this is by connecting to it via serial port (via Plink which is a command-line tool for PuTTY) and read all text lines that it prints and try to find the text string that indicates that it has been started properly. The -r option to read prevents backslash interpretation (usually used as a backslash newline pair, to continue over multiple lines or to escape the delimiters). I have apple and the count is 2 I have mango and the count is 5 I have coconut and the count is 10 I tried while read line with awk -F ',' '{print $1}', but im not getting the actual output. date, time of the log entry and other info) redirect all output into a log file; output redirection must be real-time (i. , shell variables) will not persist beyond the loop. In a bash pipeline, every element of the pipeline is executed in its own subshell . Input redirection will be used along with a while loop to read the content of a file line by line. BASH: "while read line ???" Ask Question Asked 10 years, 10 months ago. Now, follow the below steps to learn how to use the read command to read form pipe in Bash: #!/bin/bash cat names. The read attempt fails when there are no Interestingly, it works on each of the individual 29 files, but not on the joined file. exec 0>inputFile does not produce the same results, because redirecting STDIN to a file is not the same thing as redirecting a file to Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company So, I need to write a shell script that accepts a file path as an argument and uses it within the shell script. Possible solution: before entering the loop, redirect the standard input to some other file descriptor, and use read -u to read from it: #! /bin/bash while read -u3 x ; do # Read from the file descriptor 3. I changed the output filename to a hardcoded name and it worked - I was able to see the logfile and page it. Here is how I read a file row by row: while read ROW do done < file I don't use the other syntax. And it's slow, since the bash read command reads its input data byte by byte. 1. The internal field separator (IFS) is set to the empty string to preserve whitespace issues. /curl_socks_tester. If you're not seeing differences in behavior with or without the redirection, that's because do_something isn't actually reading from stdin in your tests. list | . tsv to an anonymous fifo, then passes that to stdin. sh: line 2: $1: ambiguous redirect And the script no longer works. When there is a pipeline, each command in the pipeline is executed in a subshell, and the parent shell connects all the appropriate file descriptors. x network: abc gateway: def name: xyz ip: x. /script_name < input_file, there will be zero (0) command line arguments passed to the script, hence $# will be zero. The while loop reads a line from the file, and the execution flow of the little program passes to the body of the loop. 3. However, in general, reading lines is more consistent (see the example I gave above), and it is faster because bash doesn't need to check for matching files and you don't have to wait for You have a few misconceptions. I was going to post an answer containing a short script using grep, and also mentioning that you while IFS= read line; do check=${line:0:1} done < file. Viewed 7k times 0 I understand the format below while read line do etc However, I saw this yesterday and haven't been able to figure out what var would be in the following: while read pkg var do etc Thanks. Share Improve this answer Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company As other have pointed out, awk redirection occur later. sh): #!/bin/bash while read line ; do echo "$(date): ${line}" done For example: In either of these statements it seems that if I swap out the redirect symbols, I still get the exact same result. After that, the “read command” reads & stores the input from the user and assigns it to the variable “name”. With bash process substitution you can do:. If there is any data available in input it will be read I have a long-running bash script (like a daemon), which may emit output to both stdout and stderr. So, if the code adding lines to the file is slower than the code in this script, you will reach and end point and the loop will terminate. Edited to add: In brief, the wine command's output is collected in a buffer. Run man 7 pipe and you will learn all about how pipes work. txt code of predate. Now let's turn to what goes wrong with your while read attempt. I am trying to execute a simple script to capture multiple server's details using svmatch on server names input from a file. In bash, just use space as delimiter (read -d ' '). while read line; do done <<EOT first line second line third line EOT The syntax for a while loop is. txt > find -name *. So, cmd | foo and foo < <(cmd) should be more or less equivalent. Share. @Diego: printf ' foo \n\n bar ' | bash -c 'while IFS= read -r line; I have a test file like below. txt I'm writing a Bash script where I need to look through the output of a command and do certain actions based on that output. Anything between read and do is taken as a variable. Pass members. The default would split on spc/tab/newline delimited words. Please note that read command modifies each input line by removing all leading and The problem is that the while loop is part of a pipeline. The original texts were: If you can have <backslash> in your input, do not forget to use the -r argument to read and yes, you need to use -r, because if you have an input line like box1 foo<backslash>nbar without -r you will loose the <backslash> with backticks around <backslash> and -r for the formatting of the comments. Archemar Archemar Emulating "while IFS= read -r line" in bash's C-style for loop. HandBrakeCLI reads from standard input, which steals the rest of the queue file before read line can see it. For example, run echo command 5 times or A short version—by default, if redirected to a file, stdout is fully buffered; it's written to a file only after a flush. m. The content of a file can be read line by line with this process. sh' -exec cat {} +. Tour Start here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site UPDATED#2. 6. You should not be using cut to perform a sequential iteration of each line in a file as cut was not designed to do this. Consequently, in: cut -f1 -d' ' data/socks_tested_sorted. name: xyz ip: x. One solution is to use the 'script' command recommended by user3258569 below, to have stdout flushed after every line end. for example : if you have two files, and you are not going to hard code the file name, instead you want to use the variable name H ow do I use bash while loop to repeat specific task under Linux / UNIX operating system? How do I set infinite loops using while statement? Can you provide me the while loop examples? The bash while loop is a control flow statement that allows code or commands to be executed repeatedly based on a given condition. For a single external command this is relatively easy. You can also use ssh -n instead of the redirection. 01: When this script is called with no input pipe then read will timeout after negligible 0. when the while loop reads from it), the stopped process is reawoken and it is . /file. sh > log. If that is not desired, the IFS variable has to be cleared: # Exact lines, no trimming while IFS= read Most probably you do not have bash installed and you are running your script in posix shell. Take a look at the below example. txt. I know that I could do something like: Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Yes, it is reading from standard input (stdin), the <test redirection goes to stdin. txt bob larry joe If you want to loop over the lines in the file, you need to redirect the file as input. Reason of using -t 0. $ strace -e open,dup2,pipe,write -f bash -c 'cat <<EOF > test Variable expansion with spaces are allowed in the destination of the redirection, but are ambiguous - should the space be part of the filename, or should it split the token into a filename and argument?. sh files; if you didn't need to do even that, then cat *. But you can put multiple commands in the condition part of a while loop. read data_from_subshell </tmp/fifo Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Other constructions are possible, depending on your requirements (pipe instead of redirect from process substitution if it is okay that your whole while runs in a subshell). ? Note that when using while read, the safe syntax to read lines is while IFS= read -r line; do . Obviously, there are situations where you do need to process a line at a time from a file in a shell loop, but if you just found this Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company ls -ltr | awk '{print $9}' | while read line do done in bash (and some other shells), runs the body of the while in a subshell, so state changes to the shell state (e. The redirection-operator << is used together with a tag TAG that's used to mark the end of input later: Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Visit the blog It redirects stdout but the timestamp added is of the time when the script finishes: #!/bin/bash . If you don't want to / cannot make sure that your input file has a newline at the end, you can group your cat with an echo to give the appearance of an Note the usage of IFS= and read -r according to the recommendations in BashFAQ/001: How can I read a file (data stream, variable) line-by-line (and/or field-by-field)?. Anything that foo sends to stdout will go directly to bar's stdin. Keep reading for improvements. txt and find them in your target file like so: grep -f inputs. Your awk statement:. while IFS= read -r first_line consumer do break done < <( producer ) @Netcoder's answer is good, this optimisation eliminates spurious blank lines, also allows for the last line not to have a newline, if that's how the original was. I have an external device that I need to power up and then wait for it to get started properly. While loop stops reading after the first line in Bash (6 answers) Closed 4 years ago . It indicates the interpreter to be used for executing the script, in this case, it’s Bash situated in the /bin directory. I got a problem in my Bash script, when I want to use my script it says "ambiguous redirect" at line 8 and line 43. If no leading [n] is specified the default is 0, which is normal stdin redirection. #!/bin/bash while read -r line; do echo "${line%%:*}" done < /etc/passwd actually, the only reason why I thought about this was because read prints its prompt to stderr (at least zsh's builtin read). — man cut TL;DR. bash- reading file from stdin and arguments. while read -r ITEM; do OPERATION done < <(COMMAND) Or in one line: while read -r ITEM; do OPERATION; done < <(COMMAND) Reads a single line from the standard input, or from file descriptor FD if the -u option is supplied. Follow answered Jul 13, 2013 at 9:40. With redirection, you can redirect output to a file instead of displaying it on the terminal, or instruct an application to read input from a file rather than the keyboard. text stream, by ensuring that its stdin is coming from elsewhere. But I can live without that; anyway, in options 2 and 3 above I only tried to send deepScript's stdout to __process, with the funky results copied above. The read commands read a line from the input and assign it to a variable. Note that you can also simplify the redirection as shown below. 01 seconds delay. Make sure the shebang line is: #!/bin/bash or #!/usr/bin/env bash And run the Once all lines are read from the file the bash while loop will stop. example gif. sh: #!/bin/bash while read line ; do echo "$(date): ${line}" done It seems that server output is flushed after exit of the program. Follow You can process text files line by line in bash, but it's really not recommended. x network: abc gateway: def The above 2 blocks of code, I want to write a multi-line sed command which can replace 'name' and 'network' information and The I/O redirection operators <, >, etc. Note that there's nothing stopping file names from containing newline characters. The first answer you link is difficult to scale: what if you want to read 120 lines at a time? what if you want to read N times at a time (where N is a variable)? The second answer you link is just broken, in case your stream/file contains single quotes. If you had both read and do_something reading from the same stream, In a vast majority of cases, you should avoid this. /input. This is a fail-safe feature. As Tom Fenech pointed out, bash read can omit an argument, with the unsplit output stored in REPLY. While loop stops reading after the first line in Bash (6 answers) Also, find | while read emits a newline-delimited stream, but legitimate filenames on UNIX are allowed to contain newlines. The $1 indicates the first argument passed to the script. First of all, the format of the while read do you are trying to use is: while read var; do ; done < file And not. FILE=test while read CMD; do echo "$CMD" done < "$FILE" Ask questions, find answers and collaborate at work with Stack Overflow for Teams. Improve 1. Redirect stdin to read from file while keeping stdin as input in Bash. txt" while read @snapfractalpop: you are right, the text was mangled. while list-1; do list-2; done where list-1 is one or more commands (usually one) and the loop continues while list-1 is successful (return value of zero), list-2 is the "body" of the loop. If you have already read a line of s, then after t>&s if you read a line from t, you will get the second line of the file. while read -r user; do read phpver rest < <(selectorctl --user-current --user="$user") output="$user $phpver" echo "${output//native/5. Modified 10 years, 10 months ago. I know this question appears rather frequently, but I cannot seem to find a solution on importing a . txt with the While Read command: line="input. Similarly for output file descriptors, writing a line to file descriptor s will append a line to a file as will writing a line to file descriptor t. awk -F: '{print $3}' 1. 0. Something like the example that follows. I have the while loop comment out but I feel like that is going in the right direction. If you are using awk within a loop -- you are most likely using it wrong. When space becomes available in the buffer (ie. /postgres_to_gmail. So after the while loop terminates, the while loop subshell's copy of var is discarded, and the original var of the parent (whose value is unchanged) is echoed. Since the newline character is missing at the end of your file, the read fails, so the last iteration of the while loop is skipped. In your case, this means that the only thing that your script can read from is the standard output of the cat command, I have a text file like below. c It's exactly the same as this: Does your script reference /bin/bash or /bin/sh in its hash bang line? The default system shell in Ubuntu is dash, not bash, so if you have #!/bin/sh then your script will be using a different shell than you expect. mkfifo /tmp/fifo now you can redirect the child to /tmp/fifo ( echo "This should go to STDOUT" echo "This is the data I want to pass to the parent shell" >/tmp/fifo ) & and the parent can read from there . But the read line within the while loop doesn't work as intended, that is it doesn't wait for the user response, presumably due to the while read context it is encapsulated by. To read from file descriptor 3, use read -u 3 (see Bash builtins). The while loop in Bash is a powerful construct that allows us to iterate thro Tour Start here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site while read line; do # your code done < <(cat "$@" /dev/stdin) bash file reading by line; simultaneously allow interactive stdin? 0. tmp uses -F: to specify that the internal awk variable FS (field separator) is set to the ':' character, so your fields will be what is separated Different Ways to Read File in Bash Script Using While Loop - Reading files is an essential aspect of shell scripting in the Bash environment. /header. And bash arrays (and thus expanding arr[*]) and read -a are also bash extensions. You can do so just by including the redirection operator within The whole top-level while loop has the input redirected because of done <r. Hi I have file name bonding with below entry: testnode1 eth0 tetnode2 eth2 Now I'm writing a bash script do a loop using while to put that entry in to variable then use it to below command: The redirection is "ambiguous" because your $1 is empty: You never passed your script an argument at all, because <members. Which is better between a for loop and while loop for Korn shell. I need help sending the output (stdin and stdout) from system commands to a bash function, while still accepting input from arguments. (but the actual file has 1000+ lines and many columns) apple,2 mango,5 coconut,10 I want to print this file as below. /server | . In that particular case, every time the output redirection is set up, for each echo invocation individually, the file is truncated. Tour Start here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site The -u3 command-line flag to read causes it to read from fd 3, while the 3<file2 redirection redirects fd 3 to file (opening file for reading). So in the first case, read always reads the first line of a newly opened input_file, while in the second case it keeps reading lines from the same file Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company You could read the files in a for loop as follows: for file in /tmp/log/*; do while read -r line; do echo "$line" done < "$file" done The best way to do this is to redirect the file into the loop: # Basic idea. Try Teams for free Explore Teams <is used for input redirection. From your description input. g. For further info, please see Why is using a shell loop to process text considered bad practice?, and the associated links. That solves my problem for now, but it is curious that it doesn't work on the full file. "while read line" should read directly from the file if I'm not wrong – Todoroff Commented Nov 6, 2015 at 9:07 That means all the read statements in the loop are applied successively to the same open file, so it reads all the lines of the file. Don't forget to specify the variable name into which the value should be read. I will also ls -ltr | awk '{print $9}' | while read line do done in bash (and some other shells), runs the body of the while in a subshell, so state changes to the shell state (e. e. The file descriptor table looks like this, where the standard streams are reading/writing from the TTY. You could do this with a C program but there's a far more devious way using just bash. However, for the bash-internal read command this doesn't seem to be the case. Tour Start here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site ; Why is while IFS= read used so often, instead of IFS=; while read. One way to fix this is by using Process Substitution as shown below: Thanks to n. After searching around one solution was to use "sed" with my while loop like below : #!/bin/ This is more efficient than using a separate process to skip the first line, and prevents the while loop from running in a subshell (which In the script’s first line, ‘#!’ is shebang or hashbang. And whatever is at the right side of < is NOT a command line argument. Print selected parts of lines from each FILE to standard output. To know in detail -r treats the input literally, but supplying an argument to read causes the line to be split according to IFS, a side-effect of which with the default value is to remove leading and trailing whitespace. xmzsvgoh ism aefmv iemjxnx ntpwjy kxfof ota xijejyb bnalhk kbpnt