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guptag-dev/Bash Cheat Sheet

Last active July 12, 2017 12:43
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  1. guptag-dev revised this gist Jan 24, 2017. 1 changed file with 3 additions and 0 deletions.
    3 changes: 3 additions & 0 deletions Bash Cheat Sheet
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    @@ -420,3 +420,6 @@ function returntrap {

    trap returntrap RETURN # is executed each time a shell function or a script executed with the . or source commands finishes executing

    # 8. Useful snippets

    #Command line Input
  2. guptag-dev revised this gist Jan 24, 2017. 1 changed file with 3 additions and 0 deletions.
    3 changes: 3 additions & 0 deletions Bash Cheat Sheet
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    @@ -149,6 +149,9 @@ varname=value # defines a variable
    varname=value command # defines a variable to be in the environment of a particular subprocess
    echo $varname # checks a variable's value
    varname=$(ls) # assigns command output to the varaible instead of printing it on console
    local var_name=value # assigns a local variable while default scope is global



    export VARNAME=value # defines an environment variable (will be available in subprocesses)

  3. guptag-dev revised this gist Jan 24, 2017. 1 changed file with 5 additions and 0 deletions.
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    @@ -152,6 +152,11 @@ varname=$(ls) # assigns command output to the varaible instead of

    export VARNAME=value # defines an environment variable (will be available in subprocesses)

    let counter++ # increments counter by 1
    ((counter++)) # same as last one
    let "a = 3 + 3" # evaluates the expression and assigns the value to a
    (("a = 3 + 3")) # same as last one

    array[0] = val # several ways to define an array
    array[1] = val
    array[2] = val
  4. guptag-dev revised this gist Jan 24, 2017. 1 changed file with 8 additions and 3 deletions.
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    @@ -139,12 +139,17 @@ wget <file> # downloads file
    # 2.1. Variables.


    varname=value # defines a variable
    varname=value command # defines a variable to be in the environment of a particular subprocess
    echo $varname # checks a variable's value
    echo $1 # prints value of first command line argument
    echo $2 # prints value of second command line argument
    echo $@ # prints all command line arguments
    echo $$ # prints process ID of the current shell
    echo $! # prints process ID of the most recently invoked background job
    echo $? # displays the exit status of the last command
    varname=value # defines a variable
    varname=value command # defines a variable to be in the environment of a particular subprocess
    echo $varname # checks a variable's value
    varname=$(ls) # assigns command output to the varaible instead of printing it on console

    export VARNAME=value # defines an environment variable (will be available in subprocesses)

    array[0] = val # several ways to define an array
  5. guptag-dev created this gist Jan 24, 2017.
    409 changes: 409 additions & 0 deletions Bash Cheat Sheet
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    #!/bin/bash
    #####################################################
    # Name: Bash CheatSheet for Mac OSX
    #
    # A little overlook of the Bash basics
    #
    # Usage:
    #
    # Original Author: J. Le Coupanec
    # Copies from: https://gist.github.com/LeCoupa/122b12050f5fb267e75f
    # Date: 2014/11/04
    #####################################################


    # 0. Shortcuts.


    CTRL+A # move to beginning of line
    CTRL+B # moves backward one character
    CTRL+C # halts the current command
    CTRL+D # deletes one character backward or logs out of current session, similar to exit
    CTRL+E # moves to end of line
    CTRL+F # moves forward one character
    CTRL+G # aborts the current editing command and ring the terminal bell
    CTRL+J # same as RETURN
    CTRL+K # deletes (kill) forward to end of line
    CTRL+L # clears screen and redisplay the line
    CTRL+M # same as RETURN
    CTRL+N # next line in command history
    CTRL+O # same as RETURN, then displays next line in history file
    CTRL+P # previous line in command history
    CTRL+R # searches backward
    CTRL+S # searches forward
    CTRL+T # transposes two characters
    CTRL+U # kills backward from point to the beginning of line
    CTRL+V # makes the next character typed verbatim
    CTRL+W # kills the word behind the cursor
    CTRL+X # lists the possible filename completefions of the current word
    CTRL+Y # retrieves (yank) last item killed
    CTRL+Z # stops the current command, resume with fg in the foreground or bg in the background

    DELETE # deletes one character backward
    !! # repeats the last command
    exit # logs out of current session


    # 1. Bash Basics.


    export # displays all environment variables

    echo $SHELL # displays the shell you're using
    echo $BASH_VERSION # displays bash version

    bash # if you want to use bash (type exit to go back to your normal shell)
    whereis bash # finds out where bash is on your system

    clear # clears content on window (hide displayed lines)


    # 1.1. File Commands.


    ls # lists your files
    ls -l # lists your files in 'long format', which contains the exact size of the file, who owns the file and who has the right to look at it, and when it was last modified
    ls -a # lists all files, including hidden files
    ln -s <filename> <link> # creates symbolic link to file
    touch <filename> # creates or updates your file
    cat > <filename> # places standard input into file
    more <filename> # shows the first part of a file (move with space and type q to quit)
    head <filename> # outputs the first 10 lines of file
    tail <filename> # outputs the last 10 lines of file (useful with -f option)
    emacs <filename> # lets you create and edit a file
    mv <filename1> <filename2> # moves a file
    cp <filename1> <filename2> # copies a file
    rm <filename> # removes a file
    diff <filename1> <filename2> # compares files, and shows where they differ
    wc <filename> # tells you how many lines, words and characters there are in a file
    chmod -options <filename> # lets you change the read, write, and execute permissions on your files
    gzip <filename> # compresses files
    gunzip <filename> # uncompresses files compressed by gzip
    gzcat <filename> # lets you look at gzipped file without actually having to gunzip it
    lpr <filename> # print the file
    lpq # check out the printer queue
    lprm <jobnumber> # remove something from the printer queue
    genscript # converts plain text files into postscript for printing and gives you some options for formatting
    dvips <filename> # print .dvi files (i.e. files produced by LaTeX)
    grep <pattern> <filenames> # looks for the string in the files
    grep -r <pattern> <dir> # search recursively for pattern in directory


    # 1.2. Directory Commands.


    mkdir <dirname> # makes a new directory
    cd # changes to home
    cd <dirname> # changes directory
    pwd # tells you where you currently are


    # 1.3. SSH, System Info & Network Commands.


    ssh user@host # connects to host as user
    ssh -p <port> user@host # connects to host on specified port as user
    ssh-copy-id user@host # adds your ssh key to host for user to enable a keyed or passwordless login

    whoami # returns your username
    passwd # lets you change your password
    quota -v # shows what your disk quota is
    date # shows the current date and time
    cal # shows the month's calendar
    uptime # shows current uptime
    w # displays whois online
    finger <user> # displays information about user
    uname -a # shows kernel information
    man <command> # shows the manual for specified command
    df # shows disk usage
    du <filename> # shows the disk usage of the files and directories in filename (du -s give only a total)
    last <yourUsername> # lists your last logins
    ps -u yourusername # lists your processes
    kill <PID> # kills (ends) the processes with the ID you gave
    killall <processname> # kill all processes with the name
    top # displays your currently active processes
    bg # lists stopped or background jobs ; resume a stopped job in the background
    fg # brings the most recent job in the foreground
    fg <job> # brings job to the foreground

    ping <host> # pings host and outputs results
    whois <domain> # gets whois information for domain
    dig <domain> # gets DNS information for domain
    dig -x <host> # reverses lookup host
    wget <file> # downloads file


    # 2. Basic Shell Programming.


    # 2.1. Variables.


    varname=value # defines a variable
    varname=value command # defines a variable to be in the environment of a particular subprocess
    echo $varname # checks a variable's value
    echo $$ # prints process ID of the current shell
    echo $! # prints process ID of the most recently invoked background job
    echo $? # displays the exit status of the last command
    export VARNAME=value # defines an environment variable (will be available in subprocesses)

    array[0] = val # several ways to define an array
    array[1] = val
    array[2] = val
    array=([2]=val [0]=val [1]=val)
    array(val val val)

    ${array[i]} # displays array's value for this index. If no index is supplied, array element 0 is assumed
    ${#array[i]} # to find out the length of any element in the array
    ${#array[@]} # to find out how many values there are in the array

    declare -a # the variables are treaded as arrays
    declare -f # uses funtion names only
    declare -F # displays function names without definitions
    declare -i # the variables are treaded as integers
    declare -r # makes the variables read-only
    declare -x # marks the variables for export via the environment

    ${varname:-word} # if varname exists and isn't null, return its value; otherwise return word
    ${varname:=word} # if varname exists and isn't null, return its value; otherwise set it word and then return its value
    ${varname:?message} # if varname exists and isn't null, return its value; otherwise print varname, followed by message and abort the current command or script
    ${varname:+word} # if varname exists and isn't null, return word; otherwise return null
    ${varname:offset:length} # performs substring expansion. It returns the substring of $varname starting at offset and up to length characters

    ${variable#pattern} # if the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest
    ${variable##pattern} # if the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest
    ${variable%pattern} # if the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest
    ${variable%%pattern} # if the pattern matches the end of the variable's value, delete the longest part that matches and return the rest
    ${variable/pattern/string} # the longest match to pattern in variable is replaced by string. Only the first match is replaced
    ${variable//pattern/string} # the longest match to pattern in variable is replaced by string. All matches are replaced

    ${#varname} # returns the length of the value of the variable as a character string

    *(patternlist) # matches zero or more occurences of the given patterns
    +(patternlist) # matches one or more occurences of the given patterns
    ?(patternlist) # matches zero or one occurence of the given patterns
    @(patternlist) # matches exactly one of the given patterns
    !(patternlist) # matches anything except one of the given patterns

    $(UNIX command) # command substitution: runs the command and returns standard output


    # 2.2. Functions.
    # The function refers to passed arguments by position (as if they were positional parameters), that is, $1, $2, and so forth.
    # $@ is equal to "$1" "$2"... "$N", where N is the number of positional parameters. $# holds the number of positional parameters.


    functname() {
    shell commands
    }

    unset -f functname # deletes a function definition
    declare -f # displays all defined functions in your login session


    # 2.3. Flow Control.


    statement1 && statement2 # and operator
    statement1 || statement2 # or operator

    -a # and operator inside a test conditional expression
    -o # or operator inside a test conditional expression

    str1=str2 # str1 matches str2
    str1!=str2 # str1 does not match str2
    str1<str2 # str1 is less than str2
    str1>str2 # str1 is greater than str2
    -n str1 # str1 is not null (has length greater than 0)
    -z str1 # str1 is null (has length 0)

    -a file # file exists
    -d file # file exists and is a directory
    -e file # file exists; same -a
    -f file # file exists and is a regular file (i.e., not a directory or other special type of file)
    -r file # you have read permission
    -r file # file exists and is not empty
    -w file # your have write permission
    -x file # you have execute permission on file, or directory search permission if it is a directory
    -N file # file was modified since it was last read
    -O file # you own file
    -G file # file's group ID matches yours (or one of yours, if you are in multiple groups)
    file1 -nt file2 # file1 is newer than file2
    file1 -ot file2 # file1 is older than file2

    -lt # less than
    -le # less than or equal
    -eq # equal
    -ge # greater than or equal
    -gt # greater than
    -ne # not equal

    if condition
    then
    statements
    [elif condition
    then statements...]
    [else
    statements]
    fi

    for x := 1 to 10 do
    begin
    statements
    end

    for name [in list]
    do
    statements that can use $name
    done

    for (( initialisation ; ending condition ; update ))
    do
    statements...
    done

    case expression in
    pattern1 )
    statements ;;
    pattern2 )
    statements ;;
    ...
    esac

    select name [in list]
    do
    statements that can use $name
    done

    while condition; do
    statements
    done

    until condition; do
    statements
    done


    # 3. Command-Line Processing Cycle.


    # The default order for command lookup is functions, followed by built-ins, with scripts and executables last.
    # There are three built-ins that you can use to override this order: `command`, `builtin` and `enable`.

    command # removes alias and function lookup. Only built-ins and commands found in the search path are executed
    builtin # looks up only built-in commands, ignoring functions and commands found in PATH
    enable # enables and disables shell built-ins

    eval # takes arguments and run them through the command-line processing steps all over again


    # 4. Input/Output Redirectors.


    cmd1|cmd2 # pipe; takes standard output of cmd1 as standard input to cmd2
    > file # directs standard output to file
    < file # takes standard input from file
    >> file # directs standard output to file; append to file if it already exists
    >|file # forces standard output to file even if noclobber is set
    n>|file # forces output to file from file descriptor n even if noclobber is set
    <> file # uses file as both standard input and standard output
    n<>file # uses file as both input and output for file descriptor n
    <<label # here-document
    n>file # directs file descriptor n to file
    n<file # takes file descriptor n from file
    n>>file # directs file description n to file; append to file if it already exists
    n>& # duplicates standard output to file descriptor n
    n<& # duplicates standard input from file descriptor n
    n>&m # file descriptor n is made to be a copy of the output file descriptor
    n<&m # file descriptor n is made to be a copy of the input file descriptor
    &>file # directs standard output and standard error to file
    <&- # closes the standard input
    >&- # closes the standard output
    n>&- # closes the ouput from file descriptor n
    n<&- # closes the input from file descripor n


    # 5. Process Handling.


    # To suspend a job, type CTRL+Z while it is running. You can also suspend a job with CTRL+Y.
    # This is slightly different from CTRL+Z in that the process is only stopped when it attempts to read input from terminal.
    # Of course, to interupt a job, type CTRL+C.

    myCommand & # runs job in the background and prompts back the shell

    jobs # lists all jobs (use with -l to see associated PID)

    fg # brings a background job into the foreground
    fg %+ # brings most recently invoked background job
    fg %- # brings second most recently invoked background job
    fg %N # brings job number N
    fg %string # brings job whose command begins with string
    fg %?string # brings job whose command contains string

    kill -l # returns a list of all signals on the system, by name and number
    kill PID # terminates process with specified PID

    ps # prints a line of information about the current running login shell and any processes running under it
    ps -a # selects all processes with a tty except session leaders

    trap cmd sig1 sig2 # executes a command when a signal is received by the script
    trap "" sig1 sig2 # ignores that signals
    trap - sig1 sig2 # resets the action taken when the signal is received to the default

    disown <PID|JID> # removes the process from the list of jobs

    wait # waits until all background jobs have finished


    # 6. Tips and Tricks.


    # set an alias
    cd; nano .bash_profile
    > alias gentlenode='ssh admin@gentlenode.com -p 3404' # add your alias in .bash_profile

    # to quickly go to a specific directory
    cd; nano .bashrc
    > shopt -s cdable_vars
    > export websites="/Users/mac/Documents/websites"

    source .bashrc
    cd websites


    # 7. Debugging Shell Programs.


    bash -n scriptname # don't run commands; check for syntax errors only
    set -o noexec # alternative (set option in script)

    bash -v scriptname # echo commands before running them
    set -o verbose # alternative (set option in script)

    bash -x scriptname # echo commands after command-line processing
    set -o xtrace # alternative (set option in script)

    trap 'echo $varname' EXIT # useful when you want to print out the values of variables at the point that your script exits

    function errtrap {
    es=$?
    echo "ERROR line $1: Command exited with status $es."
    }

    trap 'errtrap $LINENO' ERR # is run whenever a command in the surrounding script or function exists with non-zero status

    function dbgtrap {
    echo "badvar is $badvar"
    }

    trap dbgtrap DEBUG # causes the trap code to be executed before every statement in a function or script
    # ...section of code in which the problem occurs...
    trap - DEBUG # turn off the DEBUG trap

    function returntrap {
    echo "A return occured"
    }

    trap returntrap RETURN # is executed each time a shell function or a script executed with the . or source commands finishes executing