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

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

This is because there is not only 1 answer...

0. shell command line expansion
1. xargs dedicated tool
2. while read with some remarks
3. while read -u using dedicated fd, for interactive processing (sample)

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...

0. Read entire file as command line argument.

If your file is not too big 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)

For small amount of files (lines), this command is the lighter one.

1. xargs is the right tool

For bigger amount of files, or almost ***any*** number of lines in your input file...

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

    xargs chmod 755 <file.txt


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

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

if your command need to be run exactly 1 time by entry:

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

This is not needed for this sample, as `chmod` accept multiple files as argument, but this match the title of question.

For some special case, you could even define location of file argument in commands generateds 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 commande is done once per line.

2. while read and variants.

As OP suggest cat file.txt | while read in; do chmod 755 "$in"; done will work, but there is 2 issues:

 - `cat |` is an *useless fork*, and

 - `| while ... ;done` will become a *subshell* where environment will disapear 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 case, you may need to use

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

   For avoiding problems with stranges filenames. And maybe if you encouter problems with *`UTF-8`*:

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

• While you use STDIN for reading file.txt, your script could not be interactive (you cannot use STDIN anymore).

3. while read -u, using dedicated fd.

Syntax: while read ...;done <file.txt will redirect STDIN to file.txt. That mean, you won't be able to deal with process, until they finish.

If you plan to create interactive tool, you have to avoid use of STDIN and use some alternative file descriptor.

Constants file descriptors are: 0 for STDIN, 1 for STDOUT and 2 for STDERR. You could see them by:

    ls -l /dev/fd/

or

    ls -l /proc/self/fd/

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

For this demo, I will use fd 7:

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

Then you could use read -u 7 this way:

    while read -u 7 filename; do
        ans=
        while [ -z "$ans" ]; do
            read -p "Process file '$filename' (y/n)? " -sn1 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

    done

To close fd/7:

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

Nota: I let striked version because this syntax could be usefull, when doing many I/O with parallels process:

    mkfifo sshfifo
    exec 7> >(ssh -t user@host sh >sshfifo)
    exec 6<sshfifo

Yes.

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

This way you can avoid a cat process.

cat is almost always bad for a purpose such as this. You can read more about Useless Use of Cat.

if you have a nice selector (for example all .txt files in a dir) you could do:

for i in *.txt; do chmod 755 "$i"; done

bash for loop

or a variant of yours:

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