Differences

This shows you the differences between two versions of the page.

--- program_lang:bash [2021/10/18 10:27]
manu [backup files and folders permissions]
+++ program_lang:bash [2024/04/17 16:06] (current)
manu
@@ Line 1: / Line 1: @@
 ====== BASH ======
+===== IF =====
+if or compare text
+  if [ "$fname" = "a.txt" ] || [ "$fname" = "c.txt" ]
+  if [ $a == $b -o $b == $c -o $a == $c ]
+if and
+  if [ $a == $b -a $b == $c -a $a == $c ]
+  if [ "$fname" = "a.txt" ] && [ "$fname" = "c.txt" ]
+===== Use CSV file =====
+==== Change column order ====
+Switch column 1 and 3 serared by ;
+<cli prompt='#'>
+[root@lnx01 ~]# echo "1;2;3;4;5"  |  awk -F';' '{ t = $3 ; $3 = $1; $1 = t; print; }' OFS=';'
+;2;1;4;5
+</cli>
 ===== backup files and folders permissions =====
@@ Line 64: / Line 84: @@
 <cli prompt='$'>
-[storage@sptl064l scripts]$ basename -- $0
+[storage@lnx01l scripts]$ basename -- $0
 -bash
 </cli>
@@ Line 99: / Line 119: @@
   arr=( $(ls) ) 	Save ls output as an array of files
   ${arr[@]:s:n} 	Retrieve n elements starting at index s
+Exemple
+<cli prompt='#'>
+#!/usr/bin/bash
+declare -a Users=('user01' '1001' '1000' '' '' '/bin/bash')
+Users+=('user02' '1002' '1003' '' '' '/bin/bash')
+Users+=('user03' '1003' '4' 'Recovery user' '' '/bin/bash')
+UserNbProps=6
+let NbUsers=${#Users[@]}/$UserNbProps
+for (( i=0; i<$NbUsers; i++ ))
+do
+  let ArrayIndex=i*UserNbProps
+  Line=$(cat /etc/passwd | grep "^${Users[$ArrayIndex]}:")
+  if [ -z "$Line" ]
+  then
+    echo "# ${Users[$ArrayIndex]} user was not found"
+  else
+    Uid=$(echo $Line | cut -d: -f3)
+    Gid=$(echo $Line | cut -d: -f4)
+    UidRef=${Users[$ArrayIndex+1]}
+    GidRef=${Users[$ArrayIndex+2]}
+    if [ $Uid != $UidRef -o $Gid != $GidRef ]
+    then
+      echo "# User: ${Users[$ArrayIndex]} uid:$Uid gid:$Gid does not match reference uid:$UidRef gid:$GidRef"
+    fi
+  fi
+done
+</cli>
+===== Read files =====
+read only first line
+  head -1 myfile.txt
+read 5 last lines
+  tail -5 myfile.txt
+read the file without header (first line)
+  awk 'NR>1' myfile.txt
+===== trap =====
+The format of the trap command is:
+<cli prompt='#'>
+# trap 'command_list'  signals
+</cli>
+To ignore a signal, use two single quotes in place of the command_list:
+<cli prompt='#'>
+# trap ''  signals
+</cli>
+To reset a trap use:
+<cli prompt='#'>
+# trap ‑ signals
+</cli>
+**Example**
+<cli>
+#!/bin/bash
+#trapoff_on
+trap '' SIGINT SIGQUIT
+echo "you cannot terminate using ctrl‑c or ctrl‑\, "
+#heavy pressing go on here, cannot interrupt !
+sleep 10
+trap 'echo terminated; exit' SIGINT SIGQUIT
+#user can now interrupt
+echo "ok you can now terminate me using those keystrokes"
+sleep 10
+</cli>
+Example, to remove a file
+<cli>
+trap 'rm /tmp/hold∗.$$; exit' SIGNHUP SIGINT SIGQUIT SIGTERM
+</cli>
+=== For info ===
+To view the signals and their default actions (on an AIX machine), view the file:
+<cli prompt='$'>
+$ cat /usr/include/sys/signal.h | more
+…..
+…..
+#define SIGHUP     1    /∗ hangup, generated when terminal disconnects ∗/
+#define SIGINT     2    /∗ interrupt, generated from terminal special char ∗/
+#define SIGQUIT    3    /∗ (∗) quit, generated from terminal special char ∗/
+#define SIGILL     4    /∗ (∗) illegal instruction (not reset when caught)∗/
+#define SIGTRAP    5    /∗ (∗) trace trap (not reset when caught) ∗/
+#define SIGABRT    6    /∗ (∗) abort process ∗/
+...
+</cli>
+You can use either signal name or signal number.
+=== I have received a signal. Now what? ===
+When a signal has been received by the script, the script can do one of three actions:
+  * Ignore it and do nothing. This is probably what most scripts do without the script authors realising it.
+  * Catch the signal using trap and take appropriate action.
+  * Take the default action.
+All the above is true except for the following signals:
+  * SIGKILL (signal 9)
+  * SIGSTOP(signal 17)
+  * SIGCONT (signal 19)

wiki

User Tools

Site Tools

Differences

Page Tools