title: Bash
---

### Clever one liners

    # Search and replace in all files
    perl -p -i -e "s/from/to/g" **/*.css

### String substitutions by patterns

    STR=/path/to/foo.c

    echo ${STR%.c}      #=> "/path/to/foo"
    echo ${STR%.c}.o    #=> "/path/to/foo.o"
    echo ${STR##*.}     #=> "c" (extension)

    BASE=${SRC##*/}     #=> "foo.c" (basepath)
    DIR=${SRC%$BASE}    #=> "/path/to"

### Substitutions by regex

    echo ${STR/hi/hello}          # Replace first match
    echo ${STR//hi/hello}         # Replace all matches
    echo ${STR/#hi/hello}         # ^hi
    echo ${STR/%hi/hello}         # hi$

    echo "${STR:0:3}"             # .substr(0, 3) -- position, length
    echo "${STR:-3:3}"            # Negative position = from the right

    echo ${#line}                 # Length of $line

    [ -z "$CC" ] && CC=gcc        # CC ||= "gcc"  assignment
    ${CC:=gcc}                    # $CC || "gcc"

### Loops

    for i in /etc/rc.*; do
      echo $i
    done

### Reading input

    echo -n "Proceed? [y/n]: "
    read ans
    echo $ans

    read -n 1 ans    # Just one character

Functions
---------

### Defining functions

    myfunc() { ... }
    fuction myfunc { ... }
    fuction myfunc() { ... }

### Returning strings

    myfunc() {
        local myresult='some value'
        echo $myresult
    }

    result=$(myfunc)

### Errors

    myfunc() { return 1; }

### Arguments

    $#                          # Number of arguments
    $*                          # All args
    $1                          # First argument

Ifs - files
-----------

    # File conditions
    if [ -a FILE ]; then        # -e exists         -d directory        -f file
    fi                          # -r readable       -w writeable        -x executable
                                # -h symlink        -s size > 0

    # File comparisons
    if [ FILE1 -nt FILE2 ]      # -nt   1 more recent than 2
                                # -ot   2 more recent than 1
                                # -ef   same files

Ifs
---

    # String
    if [ -z STRING ]            # empty?
    if [ -n STRING ]            # not empty?

    # Numeric
    if [ $? -eq 0 ]             # -eq -ne -lt -le -gt -ge
                                # $? is exit status by the way

    # Etc
    if [ -o noclobber ]         # if OPTIONNAME is enabled
    if [ ! EXPR ]               # not
    if [ ONE -a TWO ]           # and
    if [ ONE -o TWO ]           # or

    # Regex
    if [[ "A" =~ "." ]]

### Numeric comparisons

    if (( $a < $b ))

### Unset variables

Assume `$FOO` is not set. Doing *this* will result in *that*:

    ${FOO:-word}                # Returns word
    ${FOO:+word}                # Returns empty, or word if set
    ${FOO:=word}                # Sets parameter to word, returns word
    ${FOO:?message}             # Echoes message and exits

    ${FOO=word}                 # : is optional in all of the above

Numeric calculations
--------------------

    $((RANDOM%=200))            # Random number 0..200
    $((a + 200))                # $ is optional

Arrays
------

    # Declaring using declare -a
    declare -a Fruits=('Apple' 'Banana' 'Orange')

    Fruits[0]="Apple"
    Fruits[1]="Banana"
    Fruits[2]="Orange"

    echo ${Fruits[0]}           # Element #0
    echo ${Fruits[@]}           # All elements, space-separated
    echo ${#Fruits[@]}          # Number of elements
    echo ${#Fruits}             # String length of the 1st element
    echo ${#Fruits[3]}          # String length of the Nth element
    echo ${Fruits[@]:3:2}       # Range (from position 3, length 2)

    Fruits=("${Fruits[@]}" "Watermelon")    # Push
    Fruits=( ${Fruits[@]/Ap*/} )            # Remove by regex match
    unset Fruits[2]                         # Remove one item
    Fruits=("${Fruits[@]}")                 # Duplicate
    Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
    lines=(`cat "logfile"`)                 # Read from file

Misc crap
---------

    command -V cd               #=> "cd is a function/alias/whatever"

### Options

    set -o noclobber            # Avoid overlay files (echo "hi" > foo)
    set -o errexit              # Used to exit upon error, avoiding cascading errors
    set -o pipefail             # Unveils hidden failures
    set -o nounset              # Exposes unset variables

### Glob options

    set -o nullglob             # Non-matching globs are removed  ('*.foo' => '')
    set -o failglob             # Non-matching globs throw errors
    set -o nocaseglob           # Case insensitive globs
    set -o dotglob              # Wildcards match dotfiles ("*.sh" => ".foo.sh")
    set -o globstar             # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')

set GLOBIGNORE as a colon-separated list of patterns to be removed from glob 
matches.

### Trap errors

    trap 'echo Error at about $LINENO' ERR

or

    traperr() {
      echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
    }

    set -o errtrace
    trap traperr ERR

References
----------

*  http://wiki.bash-hackers.org/
* http://wiki.bash-hackers.org/syntax/shellvars