Tag: bash

Pass arguments through to another program

#!/bin/bash
# print_args.sh
echo "You provided the arguments:" "$@"

# You could pass all arguments to another program like this
# myProgram "$@"

Get the number of arguments passed

#!/bin/bash
echo "You provided $# arguments"

Accessing a specific argument by index

#!/bin/bash
echo "Arg 0: $0"
echo "Arg 1: $1"
echo "Arg 2: $2"

Argument 0 is the name of the script being invoked itself.

Iterating through each argument

#!/bin/bash
for arg in "$@"
do
    echo "$arg"
done

Check arguments for specific value

#!/bin/bash
for arg in "$@"
do
    if [ "$arg" == "--help" ] || [ "$arg" == "-h" ]
    then
        echo "Help argument detected."
    fi
done

References
https://www.devdungeon.com/content/taking-command-line-arguments-bash

echo -n "Some string..."

echo -e "Some string...\c"

printf "a line without trailing linefeed"

printf "a line with trailing linefeed\n"

References
https://stackoverflow.com/questions/11193466/echo-without-newline-in-a-shell-script

Check if File Exist

FILE=/etc/resolv.conf
if test -f "$FILE"; then
    echo "$FILE exist"
fi

FILE=/etc/resolv.conf
if [ -f "$FILE" ]; then
    echo "$FILE exist"
fi

FILE=/etc/resolv.conf
if [[ -f "$FILE" ]]; then
    echo "$FILE exist"
fi

Check if Directory Exist

FILE=/etc/docker
if [ -d "$FILE" ]; then
    echo "$FILE is a directory"
fi

Check if File does Not Exist

FILE=/etc/docker
if [ ! -f "$FILE" ]; then
    echo "$FILE does not exist"
fi

Check if Multiple Files Exist

FILE=/etc/docker
if [ -f /etc/resolv.conf -a -f /etc/hosts ]; then
    echo "$FILE is a directory"
fi

FILE=/etc/docker
if [ -f /etc/resolv.conf && -f /etc/hosts ]; then
    echo "$FILE is a directory"
fi

File test operators

• -f FILE – True if the FILE exists and is a regular file (not a directory or device).
• -G FILE – True if the FILE exists and has the same group as the user running the command.
• -h FILE – True if the FILE exists and is a symbolic link.
• -g FILE – True if the FILE exists and has set-group-id (sgid) flag set.
• -k FILE – True if the FILE exists and has a sticky bit flag set.
• -L FILE – True if the FILE exists and is a symbolic link.
• -O FILE – True if the FILE exists and is owned by the user running the command.
• -p FILE – True if the FILE exists and is a pipe.
• -r FILE – True if the FILE exists and is readable.
• -S FILE – True if the FILE exists and is socket.
• -s FILE – True if the FILE exists and has nonzero size.
• -u FILE – True if the exists and set-user-id (suid) flag is set.
• -w FILE – True if the FILE exists and is writable.
• -x FILE – True if the FILE exists and is executable.

References
https://linuxize.com/post/bash-check-if-file-exists/

The return value is stored in $?. 0 indicates success, others indicates error.

some_command
if [ $? -eq 0 ]; then
    echo OK
else
    echo FAIL

some_command
retval=$?
do_something $retval
if [ $retval -ne 0 ]; then
    echo "Return code was not zero but $retval"
fi

References
https://askubuntu.com/questions/29370/how-to-check-if-a-command-succeeded

Black        0;30     Dark Gray     1;30
Red          0;31     Light Red     1;31
Green        0;32     Light Green   1;32
Brown/Orange 0;33     Yellow        1;33
Blue         0;34     Light Blue    1;34
Purple       0;35     Light Purple  1;35
Cyan         0;36     Light Cyan    1;36
Light Gray   0;37     White         1;37

RED='\033[0;31m'
NC='\033[0m' # No Color

echo -e "I ${RED}love${NC} Stack Overflow"

References
https://stackoverflow.com/questions/5947742/how-to-change-the-output-color-of-echo-in-linux
https://misc.flogisoft.com/bash/tip_colors_and_formatting

tput setaf 1; echo "this is red text"

The count of colors available to tput is given by tput colors

To see the basic 8 colors (as used by setf in urxvt terminal and setaf in xterm terminal):

Color       #define       Value       RGB
black     COLOR_BLACK       0     0, 0, 0
red       COLOR_RED         1     max,0,0
green     COLOR_GREEN       2     0,max,0
yellow    COLOR_YELLOW      3     max,max,0
blue      COLOR_BLUE        4     0,0,max
magenta   COLOR_MAGENTA     5     max,0,max
cyan      COLOR_CYAN        6     0,max,max
white     COLOR_WHITE       7     max,max,max

To print all 256 colors in the terminal, try the following one-liner:

for c in {0..255}; do tput setaf $c; tput setaf $c | cat -v; echo =$c; done

References
https://stackoverflow.com/questions/5947742/how-to-change-the-output-color-of-echo-in-linux
https://unix.stackexchange.com/questions/269077/tput-setaf-color-table-how-to-determine-color-codes

#!/bin/bash
# Basic function
print_something () {
echo Hello I am a function
}
print_something
print_something

Passing Arguments

#!/bin/bash
# Passing arguments to a function
print_something () {
echo Hello $1
}
print_something Mars
print_something Jupiter

Return Values

#!/bin/bash
# Setting a return status for a function
print_something () {
echo Hello $1
return 5
}
print_something Mars
print_something Jupiter
echo The previous function has a return value of $?

Passing Arguments

#!/bin/bash
# Passing arguments to a function
print_something () {
echo Hello $1
}
print_something Mars
print_something Jupiter

Return Values

#!/bin/bash
# Setting a return status for a function
print_something () {
echo Hello $1
return 5
}
print_something Mars
print_something Jupiter
echo The previous function has a return value of $?

the variable $? contains the return status of the previously run command or function.

References
https://ryanstutorials.net/bash-scripting-tutorial/bash-functions.php

pushd dir1
# bash commands ...
popd

References
https://unix.stackexchange.com/questions/77077/how-do-i-use-pushd-and-popd-commands
https://www.tecmint.com/pushd-and-popd-linux-filesystem-navigation/