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

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

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