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Roadmap Introduction Prompt & Alias How to Create a Shell Script? Sharp-Bang & Comments Always Use Absolute Pathname! Command Separator , ||, && Backquote/Backtick Variables Positional Parameters Misc. References
Introduction
Shell A command interpreter
For example sh, bash, csh, tcsh, kosh, zsh, …
Shell Script Whole bunch of Unix commands saved into a text file with
flow control Your shell scripting skills depends on
# Unix commands you know how well you put them together to do the right thing
Prompt & Alias
PS1="\u@\h:\w \$ “ \u the username of the current user \h the hostname \w the current working directory \$ if the effective UID is 0, a #, otherwise a $
alias v='ls -l --color=auto‘ ~/.bashrc
After editing $ source ~/.bashrc
How to Create a Shell Script? Edit hello.sh
#!/bin/sh echo "Hello World!"
Make hello.sh an executable $ chmod +x hello.sh
Run hello.sh $ ./hello.sh $ sh hello.sh
Debug hello.sh $ sh -x hello.sh
Sharp-Bang (#!) & Comments Which shell to run the script?
#! Sharp-Bang, a two-byte magic number $ cat hello.sh
#!/bin/sh # My first shell script echo "Hello World!”
Always Use Absolute Pathname! $ cat cal.sh
#!/bin/sh cal 2008
$ cat cal #!/bin/sh /usr/bin/bash
$ cat cal2.sh #!/bin/sh CAL=/usr/bin/cal $CAL 2008
Command Separator, ||, && $ cd /var/log ; tail message $ alias todo='echo ; cat -n ~/TODO ; echo‘ $ cd /var/log || { echo "Cannot change to /var/log" exit 1 } $ cat or.sh
#!/bin/sh [ -e $1 ] || exit 1 /usr/bin/less $1
$ lpr file.tmp && rm file.tmp
Backquote/Backtick
$ echo "The date is `date`" $ wget `cat z.txt` $ for i in `cat kilo.txt`; do echo “$i has `ssh $i who | wc -l` users” done
Exit Status Each Unix command returns a exit status ($?) Unlike C, 0 means true, false otherwise $ /bin/true $ echo $?
0 $ cat abc $ echo $?
1 $ touch abc $ cat abc $ echo $?
0
Exit Status Return a exit status using “exit” built-in command $ cat exit2.sh
#!/bin/sh exit 2
$ exit2.sh $ echo $?
2 $ exit-1.sh
#!/bin/sh exit -1
$ echo $? 255
Exit Status
The script returns the status of the last executable
$ cat exit3.sh #!/bin/sh /usr/bin/who | /bin/false
$ exit3.sh $ echo $?
1
Variables $ a=3
Define a variable a $ echo $a $ echo ${a} $ echo "a = $a"
$a is a reference to its value $ a =3
run a with =3 as parameter $ a= 3
# run 3 with empty environment a $ a="a b c" $ echo $a $ echo "$a"
Double quoting a variable preserves whitespaces
$ echo '$a' Single quoting a variable disables var.
referencing
$ a= Set a to a null value
$ unset a $ a=`date` $ echo $a $ a=`ls -l`
Can assign anything to a variable $ echo $a $ echo "$a"
Double quotes preserves formatting $ echo "'a' is $a" $ echo "\"a\" is $a" $ echo "\$a is $a"
Variables
$ cat user.sh #/bin/sh NUM="`who | cut -d' ' -f1 | sort | uniq | wc -l`" echo "`hostname` has $NUM users."
Positional Parameters
$ cat param.sh #!/bin/sh echo "Program = $0" echo "# of parameters = $#" echo "\$1 = $1" echo "\$2 = $2" echo "\$3 = $3" echo "\$10 = ${10}" echo "\$@ = $@" echo "\$* = $*"
$ param.sh `seq 1 10` Program = ./param.sh # of parameters = 10 $1 = 1 $2 = 2 $3 = 3 $10 = 10 $@ = 1 2 3 4 5 6 7 8 9 10 $* = 1 2 3 4 5 6 7 8 9 10
$ echo $$ Current process ID
Positional Parameters & Soft-link Trick $ ln -s param.sh newcmd.sh $ newcmd.sh `seq 1 10`
Program = ./newcmd.sh # of parameters = 10 $1 = 1 $2 = 2 $3 = 3 $10 = 10 $@ = 1 2 3 4 5 6 7 8 9 10 $* = 1 2 3 4 5 6 7 8 9 10
Branching
Syntax if <condition>; then <stmts> elif <condition>; then <stmts> else <stmts> fi
$ test.sh /etc/foo /etc/foo does not exist.
$ test.sh /etc/passwd /etc/passwd is a file.
$ test.sh /etc /etc is a directory.
How to check if a given path is a file or a directory?
How to check if a given path is a file or a directory? #!/bin/sh if test -e $1; then if [ -f $1 ]; then echo "$1 is a file." elif [ -d $1 ]; then echo "$1 is a directory." else echo "$1 has an unknown type." fi else echo "$1 does not exist." fi
[ $# -eq 1 ] || { echo "Usage: $0 file" exit 1}
If [ $# -ne 1 ]; then echo "Usage: $0 file" exit 1fi
File Test Operators
-e file exists -f regular file -s file is not zero size -d directory -b block device -c char. device -L symbolic link -r file has read permission -w write -x execute
Comparison Operators #!/bin/sh a=1 b=2 if [ "$a" -ne "$b" ]; then # Numerical comparision: 1 != 2 echo "$a and $b are not equal" else echo "$a and $b are equal" fi
if [ "$a" != "$b" ]; then # String comparision: "1" != "2” echo "$a and $b are not equal" else echo "$a and $b are equal" fi
Comparison Operators Integer Comparison Operators
-eq is equal to -ne is not equal to -gt is greater than -ge is greater than or equal to -lt is less than -le is less than or equal to
String Comparison Operators = is equal to == same as = != is not equal to -n string is not null -z string is null
-n example #!/bin/sh
# Wrong, should do [ $1 ] if [ -n $1 ]; then echo "\$1 is non-null" else echo "\$1 is null" fi
# Correct if [ -n "$1" ]; then echo "\$1 is non-null" else echo "\$1 is null" fi
Case Statement
Syntax case "$var" in value1) <stmts> ;; value2) <stmts> ;; *) <stmts> ;; esac
Case Statement - an example #!/bin/sh echo -n "Hit a key, then hit Enter: " read key case "$key" in a|b|c|d|e|f|g|h|i|j|k|l|m|n|o|p|q|r|s|t|u|v|w|x|y|z) echo "$key is a lower case." ;; A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z) echo "$key is an upper case." ;; 0|1|2|3|4|5|6|7|8|9) echo "$key is a number." ;; *) echo "$key is a punctuation." ;; esac
Case Statement - a better example #!/bin/sh read -n1 -p "Hit a key: " key echo case "$key" in [a-z]) echo "$key is a lower case." ;; [A-Z]) echo "$key is an upper case." ;; [0-9]) echo "$key is a number." ;; *) echo "$key is a punctuation." ;; esac
for loop
Syntax for <var> in <list>; do <stmts> done
For example for i in 1 2 3 4 5 6 7 8 9 10; do echo $i done
for i in `seq 1 10`; do
for loop
$ for i in /usr/*/man; do echo $i; done /usr/bin/man /usr/csshare/man /usr/kerberos/man /usr/local/man /usr/share/man
/usr/*/man gets expanded first!
while & until loop
while loop syntax while <condition>; do <stmts> done
until loop syntax until <condition>; do <stmts> done
How to keep track if a user has been logged out? #!/bin/sh user=manager while : ; do who | grep $user &> /dev/null if [ $? -ne 0 ]; then break else sleep 30 continue fi done echo "$user has logged out at `date`"
Null command always returns true $ : $ echo $?
0 Create a zero-sized file
$ : > out.txt $ cat /dev/null > out.txt $ touch out.txt
Do we care about the output?
How to keep track if a user has been logged out? #!/bin/sh user=manager while who | grep $user &> /dev/null; do sleep 30 done echo "$user has logged out at `date`"
How to keep track if a user has been logged in? #!/bin/sh user=weesan until who | grep $user &> /dev/null; do sleep 30 done echo "$user has logged in at `date`"
read #!/bin/sh while read line; do echo "$line" done < /etc/passwd
#!/bin/sh OIFS=$IFS; IFS=: while read name passwd uid gid fullname ignore; do echo "$name ($fullname)" done < /etc/passwd IFS=$OIFS
How to find the UID of “weesan”? Parse /etc/passwd line-by-line
while IFS=: read name passwd uid gid fullname ignore; do if [ "$name == "weesan" ]; then echo $uid fi done < /etc/passwd
Or, use grep & cut $ grep weesan /etc/passwd | cut -d: -f3
Or, simply $ id -u weesan
How to remove “weesan” from /etc/passwd? Parse /etc/passwd line-by-line
while IFS=: read name ignore; do if [ "$name" != "weesan" ]; then echo "$uid:$ignore” fi done < /etc/passwd
Or use grep $ grep -v weesan /etc/passwd
How to find lines starting or ending with '1'? $ cat data.txt
1a 2 * 3 411
$ grep 1 data.txt 1a 411
$ grep ^1 data.txt 1a
$ grep '1$' data.txt 411
Command Line Options
$ argv.sh -a -b -c -d 1 -e -f foo.txt Option a Option b Option c Option d with 1 Option e Unknown option: -f The rest is: foo.txt
Command Line Options $ cat argv.sh
#!/bin/sh for arg; do case "$1" in -a) echo "Option a";; -b) echo "Option b";; -c) echo "Option c";; -d) shift echo "Option d with $1" ;; -e) echo "Option e";; -*) echo "Unknown option:
$1";; *) FILE="$FILE $1";; esac shift done
echo "The rest is: $FILE"
$ cat getopts.sh #!/bin/sh while getopts "abcd:e" arg; do case "$arg" in a) echo "Option a";; b) echo "Option b";; c) echo "Option c";; d) echo "Option d with $OPTARG";; e) echo "Option e";; esac done
shift $((OPTIND - 1)) echo "The rest is: $1"
Command Line Options
$ getopts.sh -abc -d1 -ef foo.txt Option a Option b Option c Option d with 1 Option e getopts.sh: illegal option -- f The rest is: foo.txt
Here Document
#!/bin/sh cat <<EOF This is a here document. Usually it has multiple lines and long. EOF
Basename, dirname
$ dirname /boot/System.map /boot
$ basename /boot/System.map System.map
$ basename /boot/System.map .map System
Good for renaming $ a="a.txt" $ tmp="`basename $a .txt`.tmp” $ grep -v weesan $a > $tmp ; mv $tmp $a
eval
#!/bin/sh cmd='/usr/bin/grep $string $file' read -p "File: " file read -p "String: " string eval $cmd
Parameter Substitution #!/bin/sh echo "\$user is not set" echo ${user-`whoami`}
echo "\$user is set" user="foo" echo ${user-`whoami`}
echo "\$user is set and non-empty" user="foo" echo ${user-`whoami`} echo ${user:-`whoami`}
echo "\$user is set but empty" user= echo ${user-`whoami`} echo ${user:-`whoami`}
$user is not set weesan
$user is set
foo
$user is set and non-empty
foo foo
$user is set but empty
weesan
Function Call #!/bin/sh
start() { echo "Start" } stop() { echo "Stop" } restart() { echo "Restart" }
case "$1" in [Ss]tart) start;; [Ss]top) stop;; [Rr]estart) restart;; *) echo "Usage $0 start|stop|restart";; esac
Function Call - parameters
#!/bin/sh foo() { echo "# of para = $#" echo "\$0 = $0" echo "\$1 = $1" echo "\$2 = $2" echo "\$3 = $3" } foo 1 2 3
# of para = 3 $0 = ./func2.sh $1 = 1 $2 = 2 $3 = 3
find
$ find -nouser -xdev $ find /home -print0 | xargs -0 ls -l $ find /bin /usr/bin /sbin -perm -4000 -or -
perm -2000 $ find /bin /usr/bin /sbin -perm -4000 -or -
perm -2000 | xargs ls -l $ find /bin /usr/bin /sbin -perm -4000 -or -
perm -2000 -exec ls -l '{}' \;
References
Advance Bash-Script Guide http://www.tldp.org/LDP/abs/abs-guide.pdf
Unix Power Tools, 3rd Edition Shelley Powers, Jerry Peek, Tim O'Reilly, Mike
Loukides
