Linux If/elif/else/fi and case statement

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=, !=, -n, -z, -eq, -ne, -gt, etc.

String Comparison	Description
Str1 = Str2	Returns true if the strings are equal
Str1 != Str2	Returns true if the strings are not equal
-n Str1	Returns true if the string is not null
-z Str1	Returns true if the string is null
Numeric Comparison	Description
expr1 -eq expr2	Returns true if the expressions are equal
expr1 -ne expr2	Returns true if the expressions are not equal
expr1 -gt expr2	Returns true if expr1 is greater than expr2
expr1 -ge expr2	Returns true if expr1 is greater than or equal to expr2
expr1 -lt expr2	Returns true if expr1 is less than expr2
expr1 -le expr2	Returns true if expr1 is less than or equal to expr2
! expr1	Negates the result of the expression
File Conditionals	Description
-d file	True if the file is a directory
-e file	True if the file exists (note that this is not particularly portable, thus -f is generally used)
-f file	True if the provided string is a file
-g file	True if the group id is set on a file
-r file	True if the file is readable
-s file	True if the file has a non-zero size
-u	True if the user id is set on a file
-w	True if the file is writable
-x	True if the file is an executable

Examples

In this section, we'll see couple of examples of the 'if/elif/else' statement.

Exampe #1:

#!/bin/bash

# b.sh

if [ "$#" -gt 0 ]
then
  echo "There are $# args!"
fi

if [ "$1" = "arg1" ]
then
  echo "argument 1 is $1"
fi

Output:

$ bash b.sh arg1 arg2 arg3
There are 3 args!
argument 1 is arg1

Exampe #2:

#!/bin/bash

for file in *;
do
  if [ -f $f ]
  then
    echo "file : $file"
  else
    echo "dir : $file"
  fi
done

echo
echo 'find myfie1 and mydir1'

for file in *;
do
  if [ -f $file -o -d $file ]
  then
    if [ $file = 'myfile1' ]
    then
         echo $file
    elif [ $file = 'mydir1' ]
    then
         echo $file
    else
         echo '.'
    fi
  fi
done

Output:

$ bash c.sh
file : a.sh
file : b.sh
file : c.sh
file : dir1
file : dir2
file : mydir1
file : mydir2
file : myfile1
file : myfile2

find myfie1 and mydir1
.
.
.
.
.
mydir1
.
myfile1
.

Case sample #1

Syntax of bash case statement:

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

Example:

#! /bin/bash
  
ARCH=$(uname -m)
case $ARCH in
  armv6*) ARCH="armv6";;
  armv7*) ARCH="arm";;
  aarch64) ARCH="arm64";;
  x86) ARCH="386";;
  x86_64) ARCH="amd64";;
  i686) ARCH="386";;
  i386) ARCH="386";;
esac
echo "ARCH=$ARCH"

Output:

ARCH=amd64

Case sample #2

Linux kernel implements about 30 signals. Each signal identified by a number, from 1 to 31. Signals don't carry any argument and their names are mostly self explanatory. For instance SIGKILL or signal number 9 tells the program that someone tries to kill it, and SIGHUP used to signal that a terminal hangup has occurred, and it has a value of 1.

Here is an example of the "case" statement that kill a "sleep" process by sending signals and it's PID.

#! /bin/bash
  
if [ $# -lt 2 ]
then
  echo "Usage: $0 signal# PID"
  exit
fi

case $1 in
1) echo "SIGHUP"
   kill -SIGHUP $2
   ;;

2) echo "SIGINT"
   kill -SIGINT $2
   ;;

3) echo "SIGQUIT"
   kill -SIGQUIT $2
   ;;

9) echo "SIGKILL"
   kill -SIGKILL $2
   ;;

*) echo "signal# $1 not processed";;

esac

Note: $# is the count of the arguments passed to the script.

Output on terminal #1:

$ sleep 100

$ sleep 100
Quit: 3

Output on terminal #2:

$ ps aux|grep sleep|grep -v grep
ki.hong          11762   0.0  0.0  4267916    652 s000  S+    8:58AM   0:00.02 sleep 100

$ ./signal.sh 3 11762
SIGQUIT