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)