How can one write a script that accept input from either a filename argument or from stdin?

for instance, you could use less this way. one can execute less filename and equivalently cat filename | less.

is there an easy "out of the box" way to do so? or do i need to re-invent the wheel and write a bit of logic in the script?

  • @PlasmaPower As long as the question is on-topic on SU, there is no requirement to ask on a different SE site. A lot of SE sites have overlap; generally we do not need to suggest an overlapping site unless the question is either off-topic (in which case, vote to migrate) or on-topic but not getting much of a response (in which case, the asker should flag for moderator-attention/migration, not cross-post).
    – Bob
    Commented May 4, 2014 at 9:54
  • Related: How to read from file or stdin in bash? at SO
    – kenorb
    Commented May 29, 2015 at 17:12

8 Answers 8


If the file argument is the first argument to your script, test that there is an argument ($1) and that it is a file. Else read input from stdin -

So your script could contain something like this:

[ $# -ge 1 -a -f "$1" ] && input="$1" || input="-"
cat $input

e.g. then you can call the script like

./myscript.sh filename


who | ./myscript.sh

Edit Some explanation of the script:

[ $# -ge 1 -a -f "$1" ] - If at least one command line argument ($# -ge 1) AND (-a operator) the first argument is a file (-f tests if "$1" is a file) then the test result is true.

&& is the shell logical AND operator. If test is true, then assign input="$1" and cat $input will output the file.

|| is the shell logical OR operator. If the test is false, then commands following || are parsed. input is assigned to "-". The command cat - reads from the keyboard.

Summarising, if the script argument is provided and it is a file, then variable input is assigned to the file name. If there is no valid argument then cat reads from the keyboard.

  • what does && input="$1" || input="-" do and why is it outside the test operator?
    – cmo
    Commented Jul 13, 2015 at 19:29
  • I've added an edit with some explanation which I hope will help.
    – suspectus
    Commented Jul 13, 2015 at 21:31
  • What if the script has multiple arguments ($@)?
    – user4043
    Commented Apr 27, 2018 at 22:32

read reads from standard input. Redirecting it from file ( ./script <someinput ) or through pipe (dosomething | ./script) will not make it work differently.

All you have to do is to loop through all the lines in input (and it doesn't differ from iterating over the lines in file).

(sample code, processes only one line)


read var
echo $var

Will echo first line of your standard input (either through < or |).

  • thanks! i choose the other answer because it suited me better. i was wrapping another script, and i didn't wanted to loop until all input recieved (could be a lot of input... would be wasteful).
    – gilad hoch
    Commented Apr 30, 2014 at 12:00

You don't mention what shell you plan on using, so I'll assume bash, though these are pretty standard things across shells.

File Arguments

Arguments can be accessed via the variables $1-$n ($0 returns the command used to run the program). Say I have a script that just cats out n number of files with a delimiter between them:

#!/usr/bin/env bash
# Parameters:
#    1:   string delimiter between arguments 2-n
#    2-n: file(s) to cat out
for arg in ${@:2} # $@ is the array of arguments, ${@:2} slices it starting at 2.
   cat $arg
   echo $1

In this case, we are passing a file name to cat. However, if you wanted to transform the data in the file (without explicitly writing and rewriting it), you could also store the file contents in a variable:

file_contents=$(cat $filename)
[...do some stuff...]
echo $file_contents >> $new_filename

Read from stdin

As far as reading from stdin, most shells have a pretty standard read builtin, though there are differences in how prompts are specified (at the very least).

The Bash builtins man page has a pretty concise explanation of read, but I prefer the Bash Hackers page.


read var_name

Multiple Variables

To set multiple variables, just provide multiple parameter names to read:

read var1 var2 var3

read will then place one word from stdin into each variable, dumping all remaining words into the last variable.

λ read var1 var2 var3
thing1 thing2 thing3 thing4 thing5
λ echo $var1; echo $var2; echo $var3
thing3 thing4 thing5

If fewer words are entered than variables, the leftover variables will be empty (even if previously set):

λ read var1 var2 var3
thing1 thing2
λ echo $var1; echo $var2; echo $var3
# Empty line


I use -p flag often for a prompt:

read -p "Enter filename: " filename

Note: ZSH and KSH (and perhaps others) use a different syntax for prompts:

read "filename?Enter filename: " # Everything following the '?' is the prompt

Default Values

This isn't really a read trick, but I use it a lot in conjunction with read. For example:

read -p "Y/[N]: " reply

Basically, if the variable (reply) exists, return itself, but if is's empty, return the following parameter ("N").


The simplest way is to redirect stdin yourself:

if [ "$1" ] ; then exec < "$1" ; fi

Or if you prefer the more terse form:

test "$1" && exec < "$1"

Now the rest of your script can just read from stdin. Of course you can do similarly with more advanced option parsing rather than hard-coding the position of the filename as "$1".

  • exec will try to execute the argument as a command which is not what we want here.
    – Suzana
    Commented May 27, 2015 at 1:20
  • @Suzana_K: Not when it has no arguments, like here. In that case it just replaces file descriptors for the shell itself rather than a child process. Commented May 27, 2015 at 2:08
  • I copied if [ "$1" ] ; then exec < "$1" ; fi in a test script and it gives an error message because the command is unkown. Same with the terse form.
    – Suzana
    Commented May 27, 2015 at 3:05
  • 1
    @Suzana_K: What shell are you using? If that's true it's not a working implementation of the POSIX sh command/Bourne shell. Commented May 27, 2015 at 15:43
  • GNU bash 4.3.11 on Linux Mint Qiana
    – Suzana
    Commented May 27, 2015 at 15:58

You can also do:

#!/usr/bin/env bash

# Set variable input_file to either $1 or /dev/stdin, in case $1 is empty
# Note that this assumes that you are expecting the file name to operate on on $1

# You can now use "$input_file" as your file to operate on
cat "$input_file"

For more neat parameter substitution tricks in Bash, see this.

  • 1
    This is fantastic! I'm using uglifyjs < /dev/stdin and it works wonderfully!
    – fregante
    Commented Aug 19, 2015 at 7:22

use (or chain off of) something else that already behaves this way, and use "$@"

let's say i want to write a tool that will replace runs of spaces in text with tabs

tr is the most obvious way to do this, but it only accepts stdin, so we have to chain off of cat:

$ cat entab1.sh

cat "$@"|tr -s ' ' '\t'
$ cat entab1.sh|./entab1.sh

cat     "$@"|tr -s      '       '       '\t'
$ ./entab1.sh entab1.sh

cat     "$@"|tr -s      '       '       '\t'

for an example where the tool being used already behaves this way, we could reimplement this with sed instead:

$ cat entab2.sh

sed -r 's/ +/\t/g' "$@"

You can also keep it simple and use this code

When you create a script file pass_it_on.sh with this code,



You can run

cat SOMEFILE.txt | ./pass_it_on.sh

and all the contents of stdin will be simply spewed out to the screen.

Alternatively use this code to both keep a copy of stdin in a file and then spew it out to the screen.


cat > $tmpFile
cat $tmpFile    

and here is another example, maybe more readable, explained here:




echo "$VALUE"

Have fun.



The simplest way and POSIX compliant is:


which is equivalent to ${1:--}.

Then read the file as usual:

while IFS= read -r line; do
  printf '%s\n' "$line" # Or: env POSIXLY_CORRECT=1 echo "$line"
done < <(cat -- "$file")
  • POSIX doesn’t yet define done < <(cat -- "$file")-style process substitution.
    – Lucas
    Commented Oct 12, 2022 at 17:19

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