In A Shell

This is a guide to shell utilities. Mainly based on The book Linux Shell Scripting Cookbook, here is what I took.

• printf in shell: C-style, %-5s means left alignment, 5 widths; %-4.2f means float, 2 rounding, 4 width.

• color in shell: reset=0, black=30, red=31, green=32, yellow=33, blue=34, magenta=35, cyan=36, white=37. For background, plus 10 except reset. Use it like this echo -e "\e[1;31m This is red text \e[0m", e flag means escape the double quote.

• variables in shell: direct assignment, but referring with $, sometimes ${variable}; ${#variable} gives you length; use [ ] or (( )) if multiple variables trouble you.

  • # remove from beginning
  • % remove from end
  • / remove or replace

e.g. ${VAR%%PATTERN} ${VAR/PATTERN/REPLACE}

Guess what $1 & $2 and more means. $? brings the last return value.

• redirection & pipe in shell: use >, for stdout 1, stderr 2, use 1>, 2> respectively, convert stderr to stdout can also be accomplished by cmd 2>&1 output.txt or cmd &> output.txt; of course, to get away with these alloying errors, throw them into the “Black Hole”, cmd 2> /dev/null. >, <, >>, directions do not matter if you know what you are doing. tee writes stdin to file

• Arrays and Associative arrays in shell: array = (1 2 3), no comma, indexing is the same as squre bracket indexing, to retrieve all, arr[*] or arr[@]. declare -A ass_array and ass_array=([index1]=val1 [index2]=val2), there you go. But you have to ${!ass_array[*]} to get all items.

• date in shell: date +%s Unix time, yeah! You can also offer a date for operation, other specification can be found in help, like date --date "Jan 20 2001" +%A gives you exactly the weekday. This is limitless.

• function in shell: Declaration is quite C-style, however, passing args using funName arg1 arg2. read -n 2 var reads 2 vars you provide. Other arguments like -s/-d/-P/-t can also be useful and will be found in detail in help or manual. for do done, if then fi, appends ; all the time. -eq/ne/lt/le/gt/ge/ -r/w/x/f/d/e/c/b/ -a/o

• find in shell: find . -regex ".*\(\.py\|\.sh)$", another cursed string problem, escape, escape, escape. -print is handy, -type/-size are useful, caution with -delete.

• xargs and more in shell: echo "splitXsplitXsplitXsplit" | xargs -d X -n 2, amazing. wd and tr even more astonishing. paste, cut, sort, mktemp, uniq, split, diff, comm blah blah; echo ${URL%.*} or #, sometimes double, can actually save you. tail, head cd -, this is brilliant!

-sed & awk in shell: If you are familiar with vim, you will definitely feel at home.

In fact, what can really help is the help or man it self.

Do try the tldr or cheat, shellcheck helps you write.