Month: June 2012

Normally, in *nix systems like Linux, mac the file permissions are displayed as symbolic notations. So when doing a ls -l in a directory you’ll see drwxr-xr-x+ like symbolic permissions next to a directory. This is equivalent to 755 in octal permissions. I find octal permissions easier to grasp and understand. Unfortunately, there is no native switch in ls to show octal permissions. So here is a little custom command that will do the needful.

[code lang=”bash”]
ls -l | awk ‘{k=0;for(i=0;i<=8;i++)k+=((substr($1,i+2,1)~/[rwx]/)*2^(8-i));if(k)printf("%0o ",k);print}’
[/code]

This command will show octal file permissions before its symbolic representation.

If you find it impressive and would like to bookmark it, just add this line in your ~/.profile or ~/.bash_profile (depending on mac or linux) using your favorite editor (I’d use vim).

[code lang=”bash”]
alias l= "ls -la | awk ‘{k=0;for(i=0;i<=8;i++)k+=((substr(\$1,i+2,1)~/[rwx]/)*2^(8-i));if(k)printf(\" %0o \",k);print}’"
[/code]

We have just escaped $ and " with a \ in above line.

Some under-the-hood details: This command works by piping the output of ls -l to awk as its input. awk is a pattern directed scanning and processing language. For more info you can do man awk which shows:

Awk scans each input file for lines that match any of a set of patterns specified literally in prog or in one or more files specified as -f progfile. With
each pattern there can be an associated action that will be performed when a line of a file matches the pattern. Each line is matched against the pattern
portion of every pattern-action statement; the associated action is performed for each matched pattern. The file name – means the standard input. Any file
of the form var=value is treated as an assignment, not a filename, and is executed at the time it would have been opened if it were a filename. The option
-v followed by var=value is an assignment to be done before prog is executed; any number of -v options may be present. The -F fs option defines the input
field separator to be the regular expression fs.

An input line is normally made up of fields separated by white space, or by regular expression FS. The fields are denoted $1, $2, …, while $0 refers to
the entire line. If FS is null, the input line is split into one field per character.

A pattern-action statement has the form

pattern { action }

A missing { action } means print the line; a missing pattern always matches. Pattern-action statements are separated by newlines or semicolons.