Read a file line by line and execute commands: 4+ answers

Because the main usage of shell ( and others shells like bash) is to run other commands, there is not only 1 answer!!

0. Shell command line expansion
1. xargs dedicated tool
2. while read with some remarks and consideration about parallel processing
3. while read -u using dedicated fd, for interactive processing (sample)
5. running shell with inline generated script

Regarding the OP request: running chmod on all targets listed in file, xargs is the indicated tool. But for some other applications, small amount of files, etc...

1. Read entire file as command line argument.

your file is not too big (tested on my host with 128Mb file, with more than 10'000'000 lines) and
all files are well named (without spaces or other special chars like quotes),

you could use shell command line expansion. Simply:

chmod 755 $(<file.txt)

This command is the simplier one.

2. `xargs` is the right tool

For

bigger amount of files, or almost any number of lines in your input file...
files holding names that could contain spaces or special characters

For many binutils tools, like chown, chmod, rm, cp -t ...

xargs chmod 755 <file.txt

Could be used after a pipe on found files by find:

find /some/path -type f -uid 1234 -print | xargs chmod 755

If you have special chars and/or a lot of lines in file.txt.

xargs -0 chmod 755 < <(tr \\n \\0 <file.txt)

find /some/path -type f -uid 1234 -print0 | xargs -0 chmod 755

If your command need to be run exactly 1 time for each entry:

xargs -0 -n 1 chmod 755 < <(tr \\n \\0 <file.txt)

This is not needed for this sample, as chmod accepts multiple files as arguments, but this matches the title of question.

For some special cases, you could even define the location of the file argument in commands generated by xargs:

xargs -0 -I '{}' -n 1 myWrapper -arg1 -file='{}' wrapCmd < <(tr \\n \\0 <file.txt)

Test with `seq 1 5` as input

Try this:

xargs -n 1 -I{} echo Blah {} blabla {}.. < <(seq 1 5)

Blah 1 blabla 1..
Blah 2 blabla 2..
Blah 3 blabla 3..
Blah 4 blabla 4..
Blah 5 blabla 5..

where your command is executed once per line.

IMPORTANT PREAMBLE BEFORE CHAPTER 3.

Doing loop under shell is generally a bad idea! There is a lot of warning about doing loop under shell!

Before doing loop, think parallelisation and dedicated tools!!

You could use bash for interact with and administrate dedicated tools. Some samples:

Compare 1st script (Edit 2023-07-23!) with 2nd, using for and while loops, in my Swap usage by process script.
Have a look how to drawn a Mandelbrot using shell at Is bash a programming language?!

3. `while read` and variants.

For this, make sure to end the file with a newline character.

As OP suggests,

cat file.txt |
    while read in; do
        chmod 755 "$in"
    done

will work, but there are 2 issues:

cat | is a useless fork, and
| while ... ;done will become a subshell whose environment will disappear after ;done.

So this could be better written:

while read in; do
    chmod 755 "$in"
done < file.txt

But

You may be warned about $IFS and read flags:

help read

read: read [-r] ... [-d delim] ... [name ...]
    ...
    Reads a single line from the standard input... The line is split
    into fields as with word splitting, and the first word is assigned
    to the first NAME, the second word to the second NAME, and so on...
    Only the characters found in $IFS are recognized as word delimiters.
    ...
    Options:
      ...
      -d delim   continue until the first character of DELIM is read, 
                 rather than newline
      ...
      -r do not allow backslashes to escape any characters
    ...
    Exit Status:
    The return code is zero, unless end-of-file is encountered...

In some cases, you may need to use

while IFS= read -r in;do
    chmod 755 "$in"
done <file.txt

for avoiding problems with strange filenames. And maybe if you encounter problems with UTF-8:

while LANG=C IFS= read -r in ; do
    chmod 755 "$in"
done <file.txt

While you use a redirection from standard inputfor reading file.txt`, your script cannot read other input interactively (you cannot use standard input for other input anymore).

3.1 `while read` for limited number of concurrent parallel tasks

If you plan to run a big number of repetitive tasks, using multiprocessing, you could do something like:

maxProc=4
...
wait4oneTask() {
    wait -np epid
    results[epid]=$?
    ...
    unset "running[$epid]"
}
...
while read file ;do
    ...
    exec {shC_Fd}>"${tmpLoc}_${file//\//_}"
    shellcheck -f gcc "$file" >&$shC_Fd 2>&1 &
    lpid=$!
    ...
    running[lpid]=''
    ((${#running[@]}>=maxProc)) && wait4oneTask
done
while ((${#running[@]})); do
    wait4oneTask
done

This is extracted from Parallel ShellCheck script: parShellCheck.sh a sample of parallelizing process and overall statistics of collected shellcheck remarks.

4. `while read`, using dedicated `fd`.

Syntax: while read ...;done <file.txt will redirect standard input to come from file.txt. That means you won't be able to deal with processes until they finish.

This will let you use more than one input simultaneously, you could merge two files (like here: scriptReplay.sh), or maybe:

You plan to create an interactive tool, you have to avoid use of standard input and use some alternative file descriptor.

Constant file descriptors are:

0 for standard input
1 for standard output
2 for standard error.

4.1 posix shell first

You could see them by:

ls -l /dev/fd/

ls -l /proc/$$/fd/

From there, you have to choose unused numbers between 0 and 63 (more, in fact, depending on sysctl superuser tool) as your file descriptor.

For this demo, I will use file descriptor 7:

while read <&7 filename; do
    ans=
    while [ -z "$ans" ]; do
        read -p "Process file '$filename' (y/n)? " foo
        [ "$foo" ] && [ -z "${foo#[yn]}" ] && ans=$foo || echo '??'
    done
    if [ "$ans" = "y" ]; then
        echo Yes
        echo "Processing '$filename'."
    else
        echo No
    fi
done 7<file.txt

If you want to read your input file in more differents steps, you have to use:

exec 7<file.txt      # Without spaces between `7` and `<`!
# ls -l /dev/fd/

read <&7 headLine
while read <&7 filename; do
    case "$filename" in
        *'----' ) break ;;  # break loop when line end with four dashes.
    esac
    ....
done

read <&7 lastLine

exec 7<&-            # This will close file descriptor 7.
# ls -l /dev/fd/

4.2 Same under bash

Under bash, you could let him choose any free fd for you and store into a variable:
exec {varname}</path/to/input:

while read -ru ${fle} filename;do
    ans=
    while [ -z "$ans" ]; do
        read -rp "Process file '$filename' (y/n)? " -sn 1 foo
        [ "$foo" ] && [ -z "${foo/[yn]}" ] && ans=$foo || echo '??'
    done
    if [ "$ans" = "y" ]; then
        echo Yes
        echo "Processing '$filename'."
    else
        echo No
    fi
done {fle}<file.txt

exec {fle}<file.txt
# ls -l /dev/fd/
read -ru ${fle} headline

while read -ru ${fle} filename;do
    [[ -n "$filename" ]] && [[ -z ${filename//*----} ]] && break
    ....
done

read -ru ${fle} lastLine

exec {fle}<&-
# ls -l /dev/fd/

5. filtering input file for creating shell commands

sed <file.txt 's/.*/chmod 755 "&"/' | sh

This won't optimise forks, but this could be usefull for more complex (or conditional) operation:

sed <file.txt 's/.*/if [ -e "&" ];then chmod 755 "&";fi/' | sh

sed 's/.*/[ -f "&" ] \&\& echo "Processing: \\"&\\"" \&\& chmod 755 "&"/' \
    file.txt | sh

This can be very useful if sed input is a feed instead of a file. Practical sample: Using rsync log output as sed input for deleting corresponding description file when a project file are deleted. See my answer to Remove file if a file with the same name but different extension doesn't exist in another directory which differ a lot from what SO asker did expect.

How do you run a command eg chmod, for each line of a file?

Read a file line by line and execute commands: 4+ answers

1. Read entire file as command line argument.

2. xargs is the right tool

Test with seq 1 5 as input