How to read from a file or stdin in Bash?

In Perl the following code will read from file specified on command line args or from stdin:

while (<>) {
   print($_);
}

This is very convenient. I just want to know what's the simplest way to read from file or stdin in bash.

Fritz G. Mehner

The following solution reads from a file if the script is called with a file name as the first parameter $1 otherwise from standard input.

while read line
do
  echo "$line"
done < "${1:-/dev/stdin}"

The substitution ${1:-...} takes $1 if defined otherwise the file name of the standard input of the own process is used.

Perhaps the simplest solution is to redirect stdin with a merging redirect operator:

#!/bin/bash
less <&0

Stdin is file descriptor zero. The above sends the input piped to your bash script into less's stdin.

kenorb

Here is the simplest way:

#!/bin/sh
cat -

Usage:

$ echo test | sh my_script.sh
test

To assign stdin to the variable, you may use: STDIN=$(cat -) or just simply STDIN=$(cat) as operator is not necessary (as per @mklement0 comment).

To parse each line from the standard input, try the following script:

#!/bin/bash
while IFS= read -r line; do
  printf '%s\n' "$line"
done

To read from the file or stdin (if argument is not present), you can extend it to:

#!/bin/bash
file=${1--} # POSIX-compliant; ${1:--} can be used either.
while IFS= read -r line; do
  printf '%s\n' "$line" # Or: env POSIXLY_CORRECT=1 echo "$line"
done < <(cat -- "$file")

Notes:

^{- read -r - Do not treat a backslash character in any special way. Consider each backslash to be part of the input line.}

^{- Without setting IFS, by default the sequences of Space and Tab at the beginning and end of the lines are ignored (trimmed).}

^{- Use printf instead of echo to avoid printing empty lines when the line consists of a single -e, -n or -E. However there is a workaround by using env POSIXLY_CORRECT=1 echo "$line" which executes your external GNU echo which supports it. See: How do I echo "-e"?}

See: How to read stdin when no arguments are passed? at stackoverflow SE

I think this is the straight-forward way:

$ cat reader.sh
#!/bin/bash
while read line; do
  echo "reading: ${line}"
done < /dev/stdin

$ cat writer.sh
#!/bin/bash
for i in {0..5}; do
  echo "line ${i}"
done

$ ./writer.sh | ./reader.sh
reading: line 0
reading: line 1
reading: line 2
reading: line 3
reading: line 4
reading: line 5

David Souther

The echo solution adds new lines whenever IFS breaks the input stream. @fgm's answer can be modified a bit:

cat "${1:-/dev/stdin}" > "${2:-/dev/stdout}"

The Perl loop in the question reads from all the file name arguments on the command line, or from standard input if no files are specified. The answers I see all seem to process a single file or standard input if there is no file specified.

Although often derided accurately as UUOC (Useless Use of cat), there are times when cat is the best tool for the job, and it is arguable that this is one of them:

cat "$@" |
while read -r line
do
    echo "$line"
done

The only downside to this is that it creates a pipeline running in a sub-shell, so things like variable assignments in the while loop are not accessible outside the pipeline. The bash way around that is Process Substitution:

while read -r line
do
    echo "$line"
done < <(cat "$@")

This leaves the while loop running in the main shell, so variables set in the loop are accessible outside the loop.

Perl's behavior, with the code given in the OP can take none or several arguments, and if an argument is a single hyphen - this is understood as stdin. Moreover, it's always possible to have the filename with $ARGV. None of the answers given so far really mimic Perl's behavior in these respects. Here's a pure Bash possibility. The trick is to use exec appropriately.

#!/bin/bash

(($#)) || set -- -
while (($#)); do
   { [[ $1 = - ]] || exec < "$1"; } &&
   while read -r; do
      printf '%s\n' "$REPLY"
   done
   shift
done

Filename's available in $1.

If no arguments are given, we artificially set - as the first positional parameter. We then loop on the parameters. If a parameter is not -, we redirect standard input from filename with exec. If this redirection succeeds we loop with a while loop. I'm using the standard REPLY variable, and in this case you don't need to reset IFS. If you want another name, you must reset IFS like so (unless, of course, you don't want that and know what you're doing):

while IFS= read -r line; do
    printf '%s\n' "$line"
done

More accurately...

while IFS= read -r line ; do
    printf "%s\n" "$line"
done < file

Webthusiast

Please try the following code:

while IFS= read -r line; do
    echo "$line"
done < file

Code ${1:-/dev/stdin} will just understand first argument, so, how about this.

ARGS='$*'
if [ -z "$*" ]; then
  ARGS='-'
fi
eval "cat -- $ARGS" | while read line
do
   echo "$line"
done

I don't find any of these answers acceptable. In particular, the accepted answer only handles the first command line parameter and ignores the rest. The Perl program that it is trying to emulate handles all the command line parameters. So the accepted answer doesn't even answer the question. Other answers use bash extensions, add unnecessary 'cat' commands, only work for the simple case of echoing input to output, or are just unnecessarily complicated.

However, I have to give them some credit because they gave me some ideas. Here is the complete answer:

#!/bin/sh

if [ $# = 0 ]
then
        DEFAULT_INPUT_FILE=/dev/stdin
else
        DEFAULT_INPUT_FILE=
fi

# Iterates over all parameters or /dev/stdin
for FILE in "$@" $DEFAULT_INPUT_FILE
do
        while IFS= read -r LINE
        do
                # Do whatever you want with LINE here.
                echo $LINE
        done < "$FILE"
done

The following works with standard sh (Tested with dash on Debian) and is quite readable, but that's a matter of taste:

if [ -n "$1" ]; then
    cat "$1"
else
    cat
fi | commands_and_transformations

Details: If the first parameter is non-empty then cat that file, else cat standard input. Then the output of the whole if statement is processed by the commands_and_transformations.

I combined all of the above answers and created a shell function that would suit my needs. This is from a cygwin terminal of my 2 Windows10 machines where I had a shared folder between them. I need to be able to handle the following:

cat file.cpp | tx
tx < file.cpp
tx file.cpp

Where a specific filename is specified, I need to used the same filename during copy. Where input data stream has been piped thru, then i need to generate a temporary filename having the hour minute and seconds. The shared mainfolder has subfolders of the days of the week. This is for organizational purposes.

Behold, the ultimate script for my needs:

tx ()
{
  if [ $# -eq 0 ]; then
    local TMP=/tmp/tx.$(date +'%H%M%S')
    while IFS= read -r line; do
        echo "$line"
    done < /dev/stdin > $TMP
    cp $TMP //$OTHER/stargate/$(date +'%a')/
    rm -f $TMP
  else
    [ -r $1 ] && cp $1 //$OTHER/stargate/$(date +'%a')/ || echo "cannot read file"
  fi
}

If there is any way that you can see to further optimize this, I would like to know.

How about

for line in `cat`; do
    something($line);
done

来源：https://stackoverflow.com/questions/6980090/how-to-read-from-a-file-or-stdin-in-bash

标签

bash

stdin