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command line manual by JLevy

The Art of Command Line

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Fluency on the command line is a skill often neglected or considered arcane, but it improves your flexibility and productivity as an engineer in both obvious and subtle ways. This is a selection of notes and tips on using the command-line that I've found useful when working on Linux. Some tips are elementary, and some are fairly specific, sophisticated, or obscure. This page is not long, but if you can use and recall all the items here, you know a lot.

Much of this originally appeared on Quora, but given the interest there, it seems it's worth using Github, where people more talented than I can readily suggest improvements. If you see an error or something that could be better, please submit an issue or PR! (Of course please review the meta section and existing PRs/issues first.)





Everyday use

      find . -name '*.py' | xargs grep some_function
      cat hosts | xargs -I{} ssh root@{} hostname
      set -euo pipefail
      trap "echo 'error: Script failed: see failed command above'" ERR
      # do something in current dir
      (cd /some/other/dir && other-command)
      # continue in original dir
      diff /etc/hosts <(ssh somehost cat /etc/hosts)
      ControlMaster auto
      ControlPath /tmp/%r@%h:%p
      ControlPersist yes
      stat -c '%A %a %n' /etc/timezone

Processing files and data

      perl -pi.bak -e 's/old-string/new-string/g' my-files-*.txt
      # Recover backup files foo.bak -> foo:
      rename 's/\.bak$//' *.bak
      # Full rename of filenames, directories, and contents foo -> bar:
      repren --full --preserve-case --from foo --to bar .
      uconv -f utf-8 -t utf-8 -x '::Any-Lower; ::Any-NFD; [:Nonspacing Mark:] >; ::Any-NFC; ' < input.txt > output.txt

System debugging


A few examples of piecing together commands:

      cat a b | sort | uniq > c   # c is a union b
      cat a b | sort | uniq -d > c   # c is a intersect b
      cat a b b | sort | uniq -u > c   # c is set difference a - b
      awk '{ x += $3 } END { print x }' myfile
      find . -type f -ls
      cat access.log | egrep -o 'acct_id=[0-9]+' | cut -d= -f2 | sort | uniq -c | sort -rn
      function taocl() {
        curl -s |
          pandoc -f markdown -t html |
          xmlstarlet fo --html --dropdtd |
          xmlstarlet sel -t -v "(html/body/ul/li[count(p)>0])[$RANDOM mod last()+1]" |
          xmlstarlet unesc | fmt -80

Obscure but useful

MacOS X only

These are items relevant only on MacOS.

More resources


With the exception of very small tasks, code is written so others can read it. With power comes responsibility. The fact you can do something in Bash doesn't necessarily mean you should! ;)


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