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How to read a file into a variable in shell?


Question

I want to read a file and save it in variable, but I need to keep the variable and not just print out the file. How can I do this? I have written this script but it isn't quite what I needed:

#!/bin/sh
while read LINE  
do  
  echo $LINE  
done <$1  
echo 11111-----------  
echo $LINE  

In my script, I can give the file name as a parameter, so, if the file contains "aaaa", for example, it would print out this:

aaaa
11111-----

But this just prints out the file onto the screen, and I want to save it into a variable! Is there an easy way to do this?

2016/09/10
1
518
9/10/2016 11:37:40 PM

Accepted Answer

In cross-platform, lowest-common-denominator sh you use:

#!/bin/sh
value=`cat config.txt`
echo "$value"

In bash or zsh, to read a whole file into a variable without invoking cat:

#!/bin/bash
value=$(<config.txt)
echo "$value"

Invoking cat in bash or zsh to slurp a file would be considered a Useless Use of Cat.

Note that it is not necessary to quote the command substitution to preserve newlines.

See: Bash Hacker's Wiki - Command substitution - Specialities.

2017/02/15
1109
2/15/2017 7:19:55 PM

If you want to read the whole file into a variable:

#!/bin/bash
value=`cat sources.xml`
echo $value

If you want to read it line-by-line:

while read line; do    
    echo $line    
done < file.txt
2011/09/15

Two important pitfalls

which were ignored by other answers so far:

  1. Trailing newline removal from command expansion
  2. NUL character removal

Trailing newline removal from command expansion

This is a problem for the:

value="$(cat config.txt)"

type solutions, but not for read based solutions.

Command expansion removes trailing newlines:

S="$(printf "a\n")"
printf "$S" | od -tx1

Outputs:

0000000 61
0000001

This breaks the naive method of reading from files:

FILE="$(mktemp)"
printf "a\n\n" > "$FILE"
S="$(<"$FILE")"
printf "$S" | od -tx1
rm "$FILE"

POSIX workaround: append an extra char to the command expansion and remove it later:

S="$(cat $FILE; printf a)"
S="${S%a}"
printf "$S" | od -tx1

Outputs:

0000000 61 0a 0a
0000003

Almost POSIX workaround: ASCII encode. See below.

NUL character removal

There is no sane Bash way to store NUL characters in variables.

This affects both expansion and read solutions, and I don't know any good workaround for it.

Example:

printf "a\0b" | od -tx1
S="$(printf "a\0b")"
printf "$S" | od -tx1

Outputs:

0000000 61 00 62
0000003

0000000 61 62
0000002

Ha, our NUL is gone!

Workarounds:

  • ASCII encode. See below.

  • use bash extension $"" literals:

    S=$"a\0b"
    printf "$S" | od -tx1
    

    Only works for literals, so not useful for reading from files.

Workaround for the pitfalls

Store an uuencode base64 encoded version of the file in the variable, and decode before every usage:

FILE="$(mktemp)"
printf "a\0\n" > "$FILE"
S="$(uuencode -m "$FILE" /dev/stdout)"
uudecode -o /dev/stdout <(printf "$S") | od -tx1
rm "$FILE"

Output:

0000000 61 00 0a
0000003

uuencode and udecode are POSIX 7 but not in Ubuntu 12.04 by default (sharutils package)... I don't see a POSIX 7 alternative for the bash process <() substitution extension except writing to another file...

Of course, this is slow and inconvenient, so I guess the real answer is: don't use Bash if the input file may contain NUL characters.


this works for me: v=$(cat <file_path>) echo $v

2018/08/30

As Ciro Santilli notes using command substitutions will drop trailing newlines. Their workaround adding trailing characters is great, but after using it for quite some time I decided I needed a solution that didn't use command substitution at all.

My approach now uses read along with the printf builtin's -v flag in order to read the contents of stdin directly into a variable.

# Reads stdin into a variable, accounting for trailing newlines. Avoids needing a subshell or
# command substitution.
read_input() {
  # Use unusual variable names to avoid colliding with a variable name
  # the user might pass in (notably "contents")
  : "${1:?Must provide a variable to read into}"
  if [[ "$1" == '_line' || "$1" == '_contents' ]]; then
    echo "Cannot store contents to $1, use a different name." >&2
    return 1
  fi

  local _line _contents
   while read -r _line; do
     _contents="${_contents}${_line}"$'\n'
   done
   _contents="${_contents}${_line}" # capture any content after the last newline
   printf -v "$1" '%s' "$_contents"
}

This supports inputs with or without trailing newlines.

Example usage:

$ read_input file_contents < /tmp/file
# $file_contents now contains the contents of /tmp/file
2018/04/26

With bash you may use read like tis:

#!/usr/bin/env bash

{ IFS= read -rd '' value <config.txt;} 2>/dev/null

printf '%s' "$value"

Notice that:

  • The last newline is preserved.

  • The stderr is silenced to /dev/null by redirecting the whole commands block, but the return status of the read command is preserved, if one needed to handle read error conditions.

2020/07/11

Source: https://stackoverflow.com/questions/7427262
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