[bash] 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...

1. shell command line expansion
2. xargs dedicated tool
3. while read with some remarks
4. 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...

1. 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.

2. 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.

3. 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).

4. 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