bash(1)

Expansion

Generator

# generate sequence from n to m
-{n..m}
-# generate sequence from n to m step by s
-{n..m..s}
-
-# expand cartesian product
-{a,b}{c,d}
-

Parameter

# default value
-bar=${foo:-some_val}  # if $foo set, then bar=$foo else bar=some_val
-
-# alternate value
-bar=${foo:+bla $foo}  # if $foo set, then bar="bla $foo" else bar=""
-
-# check param set
-bar=${foo:?msg}  # if $foo set, then bar=$foo else exit and print msg
-
-# indirect
-FOO=foo
-BAR=FOO
-bar=${!BAR}  # deref value of BAR -> bar=$FOO
-
-# prefix
-${foo#prefix}  # remove prefix when expanding $foo
-# suffix
-${foo%suffix}  # remove suffix when expanding $foo
-
-# substitute
-${foo/pattern/string}  # replace pattern with string when expanding foo
-# pattern starts with
-# '/'   replace all occurences of pattern
-# '#'   pattern match at beginning
-# '%'   pattern match at end
-
-# set programmatically with priintf builtin
-printf -v "VAR1" "abc"
-NAME=VAR2
-printf -v "$NAME" "%s" "def"
-

-
Note: prefix/suffix/pattern are expanded as pathnames.
-

Pathname

*           match any string
-?           match any single char
-\\          match backslash
-[abc]       match any char of 'a' 'b' 'c'
-[a-z]       match any char between 'a' - 'z'
-[^ab]       negate, match all not 'a' 'b'
-[:class:]   match any char in class, available:
-              alnum,alpha,ascii,blank,cntrl,digit,graph,lower,
-              print,punct,space,upper,word,xdigit
-

With extglob shell option enabled it is possible to have more powerful -patterns. In the following pattern-list is one ore more patterns separated -by | char.

?(pattern-list)   matches zero or one occurrence of the given patterns
-*(pattern-list)   matches zero or more occurrences of the given patterns
-+(pattern-list)   matches one or more occurrences of the given patterns
-@(pattern-list)   matches one of the given patterns
-!(pattern-list)   matches anything except one of the given patterns
-

-
Note: shopt -s extglob/shopt -u extglob to enable/disable extglob -option.
-

I/O redirection

-
Note: The trick with bash I/O redirection is to interpret from left-to-right.
-

# stdout & stderr to file
-command >file 2>&1
-# equivalent
-command &>file
-
-# stderr to stdout & stdout to file
-command 2>&1 >file
-

-
The article Bash One-Liners Explained, Part III: All about -redirections -contains some nice visualization to explain bash redirections.
-

Explanation

j>&i
-

Duplicate fd i to fd j, making j a copy of i. See dup2(2).

Example:

command 2>&1 >file
-

duplicate fd 1 to fd 2, effectively redirecting stderr to stdout
redirect stdout to file

Process substitution (ref)

Process substitution allows to redirect the stdout of multiple processes at -once.

vim -d <(grep foo bar) <(grep foo moose)
-

Command grouping

Execute commands in a group with or without subshell. Can be used to easily -redirect stdout/stderr of all commands in the group into one file.

# Group commands without subshell.
-v=abc ; { v=foo; echo $v; } ; echo $v
-# foo
-# foo
-
-# Group commands with subshell.
-v=abc ; ( v=foo; echo $v; ) ; echo $v
-# foo
-# abc
-

Argument parsing with `getopts`

The getopts builtin uses following global variables:

OPTARG, value of last option argument
OPTIND, index of the next argument to process (user must reset)
OPTERR, display errors if set to 1

getopts <optstring> <param> [<args>]
-

<optstring> specifies the names of supported options, eg f:c -
- f: means -f option with an argument
- c means -c option without an argument
-
<param> specifies a variable name which getopts fills with the last parsed option argument
<args> optionally specify argument string to parse, by default getopts parses $@

Example

#!/bin/bash
-function parse_args() {
-    while getopts "f:c" PARAM; do
-        case $PARAM in
-            f) echo "GOT -f $OPTARG";;
-            c) echo "GOT -c";;
-            *) echo "ERR: print usage"; exit 1;;
-        esac
-    done
-    # users responsibility to reset OPTIND
-    OPTIND=1
-}
-
-parse_args -f xxx -c
-parse_args -f yyy
-

Regular Expressions

Bash supports regular expression matching with the binary operator =~. -The match results can be accessed via the $BASH_REMATCH variable:

${BASH_REMATCH[0]} contains the full match
${BASH_REMATCH[1]} contains match of the first capture group

INPUT='title foo : 1234'
-REGEX='^title (.+) : ([0-9]+)$'
-if [[ $INPUT =~ $REGEX ]]; then
-    echo "${BASH_REMATCH[0]}"    # title foo : 1234
-    echo "${BASH_REMATCH[1]}"    # foo
-    echo "${BASH_REMATCH[2]}"    # 1234
-fi
-

-
Caution: When specifying a regex in the [[ ]] block directly, quotes will be treated as part of the pattern. -[[ $INPUT =~ "foo" ]] will match against "foo" not foo!
-

Completion

The complete builtin is used to interact with the completion system.

complete                    # print currently installed completion handler
-complete -F <func> <cmd>    # install <func> as completion handler for <cmd>
-complete -r <cmd>           # uninstall completion handler for <cmd>
-

Variables available in completion functions:

# in
-$1              # <cmd>
-$2              # current word
-$3              # privous word
-
-COMP_WORDS      # array with current command line words
-COMP_CWORD      # index into COMP_WORDS with current cursor position
-
-# out
-COMPREPLY       # array with possible completions
-

The compgen builtin is used to generate possible matches by comparing word -against words generated by option.

compgen <option> <word>
-
-# usefule options:
-# -W <list>    specify list of possible completions
-# -d           generate list with dirs
-# -f           generate list with files
-# -u           generate list with users
-# -e           generate list with exported variables
-
-# compare "f" against words "foo" "foobar" "bar" and generate matches
-compgen -W "foo foobar bar" "f"
-
-# compare "hom" against file/dir names and generate matches
-compgen -d -f "hom"
-

Example

Skeleton to copy/paste for writing simple completions.

Assume a program foo with the following interface:

foo -c green|red|blue -s low|high -f <file> -h
-

The completion handler could be implemented as follows:

function _foo() {
-    local curr=$2
-    local prev=$3
-
-    local opts="-c -s -f -h"
-    case $prev in
-        -c) COMPREPLY=( $(compgen -W "green red blue" -- $curr) );;
-        -s) COMPREPLY=( $(compgen -W "low high" -- $curr) );;
-        -f) COMPREPLY=( $(compgen -f -- $curr) );;
-        *)  COMPREPLY=( $(compgen -W "$opts" -- $curr) );;
-    esac
-}
-
-complete -F _foo foo
-

- -

Notes

Process substitution (ref)